General Science by Bertha M. Clark

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Title: General Science

Author: Bertha M. Clark

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GENERAL SCIENCE




BY

BERTHA M. CLARK, PH.D.


HEAD OF THE SCIENCE DEPARTMENT

WILLIAM PENN HIGH SCHOOL FOR GIRLS, PHILADELPHIA






NEW YORK - CINCINNATI - CHICAGO

AMERICAN BOOK COMPANY

1912





PREFACE


This book is not intended to prepare for college entrance
examinations; it will not, in fact, prepare for any of the present-day
stock examinations in physics, chemistry, or hygiene, but it should
prepare the thoughtful reader to meet wisely and actively some of
life's important problems, and should enable him to pass muster on the
principles and theories underlying scientific, and therefore economic,
management, whether in the shop or in the home.

We hear a great deal about the conservation of our natural resources,
such as forests and waterways; it is hoped that this book will show
the vital importance of the conservation of human strength and health,
and the irreparable loss to society of energy uselessly dissipated,
either in idle worry or in aimless activity. Most of us would reproach
ourselves for lack of shrewdness if we spent for any article more than
it was worth, yet few of us consider that we daily expend on domestic
and business tasks an amount of energy far in excess of that actually
required. The farmer who flails his grain instead of threshing it
wastes time and energy; the housewife who washes with her hands alone
and does not aid herself by the use of washing machine and proper
bleaching agents dissipates energy sadly needed for other duties.

The Chapter on machines is intended not only as a stimulus to the
invention of further labor-saving devices, but also as an eye opener
to those who, in the future struggle for existence, must perforce go
to the wall unless they understand how to make use of contrivances
whereby man's limited physical strength is made effective for larger
tasks.

The Chapter on musical instruments is more detailed than seems
warranted at first sight; but interest in orchestral instruments is
real and general, and there is a persistent desire for intelligent
information relative to musical instruments. The child of the laborer
as well as the child of the merchant finds it possible to attend some
of the weekly orchestral concerts, with their tiers of cheap seats,
and nothing adds more to the enjoyment and instruction of such hours
than an intimate acquaintance with the leading instruments. Unless
this is given in the public schools, a large percentage of mankind is
deprived of it, and it is for this reason that so large a share of the
treatment of sound has been devoted to musical instruments.

The treatment of electricity is more theoretical than that used in
preceding Chapters, but the subject does not lend itself readily to
popular presentation; and, moreover, it is assumed that the
information and training acquired in the previous work will give the
pupil power to understand the more advanced thought and method.

The real value of a book depends not so much upon the information
given as upon the permanent interest stimulated and the initiative
aroused. The youthful mind, and indeed the average adult mind as
well, is singularly non-logical and incapable of continued
concentration, and loses interest under too consecutive thought and
sustained style. For this reason the author has sacrificed at times
detail to general effect, logical development to present-day interest
and facts, and has made use of a popular, light style of writing as
well as of the more formal and logical style common to books of
science.

No claim is made to originality in subject matter. The actual facts,
theories, and principles used are such as have been presented in
previous textbooks of science, but the manner and sequence of
presentation are new and, so far as I know, untried elsewhere. These
are such as in my experience have aroused the greatest interest and
initiative, and such as have at the same time given the maximum
benefit from the informational standpoint. In no case, however, is
mental training sacrificed to information; but mental development is
sought through the student's willing and interested participation in
the actual daily happenings of the home and the shop and the field,
rather than through formal recitations and laboratory experiments.

Practical laboratory work in connection with the study of this book is
provided for in my _Laboratory Manual in General Science_, which
contains directions for a series of experiments designed to make the
pupil familiar with the facts and theories discussed in the textbook.

I have sought and have gained help from many of the standard
textbooks, new and old. The following firms have kindly placed cuts
at my disposal, and have thus materially aided in the preparation of
the illustrations: American Radiator Company; Commercial Museum,
Philadelphia; General Electric Company; Hershey Chocolate Company;
_Scientific American_; The Goulds Manufacturing Company; Victor
Talking Machine Company. Acknowledgment is also due to Professor Alvin
Davison for figures 19, 23, 29, 142, and 161.

Mr. W.D. Lewis, Principal of the William Penn High School, has read
the manuscript and has given me the benefit of his experience and
interest. Miss. Helen Hill, librarian of the same school, has been of
invaluable service as regards suggestions and proof reading. Miss.
Droege, of the Baldwin School, Bryn Mawr, has also been of very great
service. Practically all of my assistants have given of their time and
skill to the preparation of the work, but the list is too long for
individual mention.

BERTHA M. CLARK.

WILLIAM PENN HIGH SCHOOL.




CONTENTS


CHAPTER

I. HEAT

II. TEMPERATURE AND HEAT

III. OTHER FACTS ABOUT HEAT

IV. BURNING OR OXIDATION

V. FOOD

VI. WATER

VII. AIR

VIII. GENERAL PROPERTIES OF GASES

IX. INVISIBLE OBJECTS

X. LIGHT

XI. REFRACTION

XII. PHOTOGRAPHY

XIII. COLOR

XIV. HEAT AND LIGHT AS COMPANIONS

XV. ARTIFICIAL LIGHTING

XVI. MAN'S WAY OF HELPING HIMSELF

XVII. THE POWER BEHIND THE ENGINE

XVIII. PUMPS AND THEIR VALUE TO MAN

XIX. THE WATER PROBLEM OF A LARGE CITY

XX. MAN'S CONQUEST OF SUBSTANCES

XXI. FERMENTATION

XXII. BLEACHING

XXIII. DYEING

XXIV. CHEMICALS AS DISINFECTANTS AND PRESERVATIVES

XXV. DRUGS AND PATENT MEDICINES

XXVI. NITROGEN AND ITS RELATION TO PLANTS

XXVII. SOUND

XXVIII. MUSICAL INSTRUMENTS

XXIX. SPEAKING AND HEARING

XXX. ELECTRICITY

XXXI. SOME USES OF ELECTRICITY

XXXII. MODERN ELECTRICAL INVENTIONS

XXXIII. MAGNETS AND CURRENTS

XXXIV. HOW ELECTRICITY MAY BE MEASURED

XXXV. HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE


INDEX


GENERAL SCIENCE




CHAPTER I

HEAT


I. Value of Fire. Every day, uncontrolled fire wipes out human
lives and destroys vast amounts of property; every day, fire,
controlled and regulated in stove and furnace, cooks our food and
warms our houses. Fire melts ore and allows of the forging of iron, as
in the blacksmith's shop, and of the fashioning of innumerable objects
serviceable to man. Heated boilers change water into the steam which
drives our engines on land and sea. Heat causes rain and wind, fog and
cloud; heat enables vegetation to grow and thus indirectly provides
our food. Whether heat comes directly from the sun or from artificial
sources such as coal, wood, oil, or electricity, it is vitally
connected with our daily life, and for this reason the facts and
theories relative to it are among the most important that can be
studied. Heat, if properly regulated and controlled, would never be
injurious to man; hence in the following paragraphs heat will be
considered merely in its helpful capacity.

2. General Effect of Heat. _Expansion and Contraction_. One of the
best-known effects of heat is the change which it causes in the size
of a substance. Every housewife knows that if a kettle is filled with
cold water to begin with, there will be an overflow as soon as the
water becomes heated. Heat causes not only water, but all other
liquids, to occupy more space, or to expand, and in some cases the
expansion, or increase in size, is surprisingly large. For example, if
100 pints of ice water is heated in a kettle, the 100 pints will
steadily expand until, at the boiling point, it will occupy as much
space as 104 pints of ice water.

The expansion of water can be easily shown by heating a flask (Fig. I)
filled with water and closed by a cork through which a narrow tube
passes. As the water is heated, it expands and forces its way up the
narrow tube. If the heat is removed, the liquid cools, contracts, and
slowly falls in the tube, resuming in time its original size or
volume. A similar observation can be made with alcohol, mercury, or
any other convenient liquid.

[Illustration: FIG. 1.--As the water becomes warmer it expands and
rise in the narrow tube.]

Not only liquids are affected by heat and cold, but solids also are
subject to similar changes. A metal ball which when cool will just
slip through a ring (Fig. 2) will, when heated, be too large to slip
through the ring. Telegraph and telephone wires which in winter are
stretched taut from pole to pole, sag in hot weather and are much too
long. In summer they are exposed to the fierce rays of the sun, become
strongly heated, and expand sufficiently to sag. If the wires were
stretched taut in the summer, there would not be sufficient leeway for
the contraction which accompanies cold weather, and in winter they
would snap.

[Illustration: FIG. 2--When the ball is heated, it become too large to
slip through the ring.]

Air expands greatly when heated (Fig. 3), but since air is practically
invisible, we are not ordinarily conscious of any change in it. The
expansion of air can be readily shown by putting a drop of ink in a
thin glass tube, inserting the tube in the cork of a flask, and
applying heat to the flask (Fig. 4). The ink is forced up the tube by
the expanding air. Even the warmth of the hand is generally sufficient
to cause the drop to rise steadily in the tube. The rise of the drop
of ink shows that the air in the flask occupies more space than
formerly, and since the quantity of air has not changed, each cubic
inch of space must hold less warm air than| it held of cold air; that
is, one cubic inch of warm air weighs less than one cubic inch of cold
air, or warm air is less dense than cold air. All gases, if not
confined, expand when heated and contract as they cool. Heat, in
general, causes substances to expand or become less dense.

[Illustration: FIG. 3--As the air in _A_ is heated, it expands and
escapes in the form of bubbles.]

3. Amount of Expansion and Contraction. While most substances expand
when heated and contract when cooled, they are not all affected
equally by the same changes in temperature. Alcohol expands more than
water, and water more than mercury. Steel wire which measures 1/4 mile
on a snowy day will gain 25 inches in length on a warm summer day, and
an aluminum wire under the same conditions would gain 50 inches in
length.

[Illustration: FIG. 4.--As the air in _A_ is heated, it expands and
forces the drop of ink up the tube.]

4. Advantages and Disadvantages of Expansion and Contraction. We owe
the snug fit of metal tires and bands to the expansion and contraction
resulting from heating and cooling. The tire of a wagon wheel is made
slightly smaller than the wheel which it is to protect; it is then
put into a very hot fire and heated until it has expanded sufficiently
to slip on the wheel. As the tire cools it contracts and fits the
wheel closely.

In a railroad, spaces are usually left between consecutive rails in
order to allow for expansion during the summer.

The unsightly cracks and humps in cement floors are sometimes due to
the expansion resulting from heat (Fig. 5). Cracking from this cause
can frequently be avoided by cutting the soft cement into squares, the
spaces between them giving opportunity for expansion just as do the
spaces between the rails of railroads.

[Illustration: FIG. 5: A cement walk broken by expansion due to sun
heat.]

In the construction of long wire fences provision must be made for
tightening the wire in summer, otherwise great sagging would occur.

Heat plays an important part in the splitting of rocks and in the
formation of débris. Rocks in exposed places are greatly affected by
changes in temperature, and in regions where the changes in
temperature are sudden, severe, and frequent, the rocks are not able
to withstand the strain of expansion and contraction, and as a result
crack and split. In the Sahara Desert much crumbling of the rock into
sand has been caused by the intense heat of the day followed by the
sharp frost of night. The heat of the day causes the rocks to expand,
and the cold of night causes them to contract, and these two forces
constantly at work loosen the grains of the rock and force them out of
place, thus producing crumbling.

[Illustration: FIG. 6.--Splitting and crumbling of rock caused by
alternating heat and cold.]

The surface of the rock is the most exposed part, and during the day
the surface, heated by the sun's rays, expands and becomes too large
for the interior, and crumbling and splitting result from the strain.
With the sudden fall of temperature in the late afternoon and night,
the surface of the rock becomes greatly chilled and colder than the
rock beneath; the surface rock therefore contracts and shrinks more
than the underlying rock, and again crumbling results (Fig. 6).

[Illustration: FIG. 7.--Debris formed from crumbled rock.]

On bare mountains, the heating and cooling effects of the sun are very
striking(Fig. 7); the surface of many a mountain peak is covered with
cracked rock so insecure that a touch or step will dislodge the
fragments and start them down the mountain slope. The lower levels of
mountains are frequently buried several feet under débris which has
been formed in this way from higher peaks, and which has slowly
accumulated at the lower levels.

5. Temperature. When an object feels hot to the touch, we say that
it has a high temperature; when it feels cold to the touch, that it
has a low temperature; but we are not accurate judges of heat. Ice
water seems comparatively warm after eating ice cream, and yet we know
that ice water is by no means warm. A room may seem warm to a person
who has been walking in the cold air, while it may feel decidedly cold
to some one who has come from a warmer room. If the hand is cold,
lukewarm water feels hot, but if the hand has been in very hot water
and is then transferred to lukewarm water, the latter will seem cold.
We see that the sensation or feeling of warmth is not an accurate
guide to the temperature of a substance; and yet until 1592, one
hundred years after the discovery of America, people relied solely
upon their sensations for the measurement of temperature. Very hot
substances cannot be touched without injury, and hence inconvenience
as well as the necessity for accuracy led to the invention of the
thermometer, an instrument whose operation depends upon the fact that
most substances expand when heated and contract when cooled.

[Illustration: FIG. 8.--Making a thermometer.]

6. The Thermometer. The modern thermometer consists of a glass tube
at the lower end of which is a bulb filled with mercury or colored
alcohol (Fig. 8). After the bulb has been filled with the mercury, it
is placed in a beaker of water and the water is heated by a Bunsen
burner. As the water becomes warmer and warmer the level of the
mercury in the tube steadily rises until the water boils, when the
level remains stationary (Fig. 9). A scratch is made on the tube to
indicate the point to which the mercury rises when the bulb is placed
in boiling water, and this point is marked 212°. The tube is then
removed from the boiling water, and after cooling for a few minutes,
it is placed in a vessel containing finely chopped ice (Fig. 10). The
mercury column falls rapidly, but finally remains stationary, and at
this level another scratch is made on the tube and the point is marked
32°. The space between these two points, which represent the
temperatures of boiling water and of melting ice, is divided into 180
equal parts called degrees. The thermometer in use in the United
States is marked in this way and is called the Fahrenheit thermometer
after its designer. Before the degrees are etched on the thermometer
the open end of the tube is sealed.

[Illustration: FIG. 9.--Determining one of the fixed points of a
thermometer.]

The Centigrade thermometer, in use in foreign countries and in all
scientific work, is similar to the Fahrenheit except that the fixed
points are marked 100° and 0°, and the interval between the points is
divided into 100 equal parts instead of into 180.

_The boiling point of water is 212° F. or 100° C_.

_The melting point of ice is 32° F. or 0° C_.

Glass thermometers of the above type are the ones most generally used,
but there are many different types for special purposes.

[Illustration: FIG. 10.--Determining the lower fixed point of a
thermometer.]

7. Some Uses of a Thermometer. One of the chief values of a
thermometer is the service it has rendered to medicine. If a
thermometer is held for a few minutes under the tongue of a normal,
healthy person, the mercury will rise to about 98.4° F. If the
temperature of the body registers several degrees above or below this
point, a physician should be consulted immediately. The temperature of
the body is a trustworthy indicator of general physical condition;
hence in all hospitals the temperature of patients is carefully taken
at stated intervals.

Commercially, temperature readings are extremely important. In sugar
refineries the temperature of the heated liquids is observed most
carefully, since a difference in temperature, however slight, affects
not only the general appearance of sugars and sirups, but the quality
as well. The many varieties of steel likewise show the influence which
heat may have on the nature of a substance. By observation and tedious
experimentation it has been found that if hardened steel is heated to
about 450° F. and quickly cooled, it gives the fine cutting edge of
razors; if it is heated to about 500° F. and then cooled, the metal is
much coarser and is suitable for shears and farm implements; while if
it is heated but 50° F. higher, that is, to 550° F., it gives the fine
elastic steel of watch springs.

[Illustration: FIG. 11.--A well-made commercial thermometer.]

A thermometer could be put to good use in every kitchen; the
inexperienced housekeeper who cannot judge of the "heat" of the oven
would be saved bad bread, etc., if the thermometer were a part of her
equipment. The thermometer can also be used in detecting adulterants.
Butter should melt at 94° F.; if it does not, you may be sure that it
is adulterated with suet or other cheap fat. Olive oil should be a
clear liquid above 75° F.; if, above this temperature, it looks
cloudy, you may be sure that it too is adulterated with fat.

8. Methods of Heating Buildings. _Open Fireplaces and Stoves._
Before the time of stoves and furnaces, man heated his modest dwelling
by open fires alone. The burning logs gave warmth to the cabin and
served as a primitive cooking agent; and the smoke which usually
accompanies burning bodies was carried away by means of the chimney.
But in an open fireplace much heat escapes with the smoke and is lost,
and only a small portion streams into the room and gives warmth.

When fuel is placed in an open fireplace (Fig. 12) and lighted, the
air immediately surrounding the fire becomes warmer and, because of
expansion, becomes lighter than the cold air above. The cold air,
being heavier, falls and forces the warmer air upward, and along with
the warm air goes the disagreeable smoke. The fall of the colder and
heavier air, and the rise of the warmer and hence lighter air, is
similar to the exchange which takes place when water is poured on oil;
the water, being heavier than oil, sinks to the bottom and forces the
oil to the surface. The warmer air which escapes up the chimney
carries with it the disagreeable smoke, and when all the smoke is got
rid of in this way, the chimney is said to draw well.

[Illustration: FIG. 12.--The open fireplace as an early method of
heating.]

As the air is heated by the fire it expands, and is pushed up the
chimney by the cold air which is constantly entering through loose
windows and doors. Open fireplaces are very healthful because the air
which is driven out is impure, while the air which rushes in is fresh
and brings oxygen to the human being.

But open fireplaces, while pleasant to look at, are not efficient for
either heating or cooking. The possibilities for the latter are
especially limited, and the invention of stoves was a great advance in
efficiency, economy, and comfort. A stove is a receptacle for fire,
provided with a definite inlet for air and a definite outlet for
smoke, and able to radiate into the room most of the heat produced
from the fire which burns within. The inlet, or draft, admits enough
air to cause the fire to burn brightly or slowly as the case may be.
If we wish a hot fire, the draft is opened wide and enough air enters
to produce a strong glow. If we wish a low fire, the inlet is only
partially opened, and just enough air enters to keep the fuel
smoldering.

When the fire is started, the damper should be opened wide in order to
allow the escape of smoke; but after the fire is well started there is
less smoke, and the damper may be partly closed. If the damper is kept
open, coal is rapidly consumed, and the additional heat passes out
through the chimney, and is lost to use.

9. Furnaces. _Hot Air_. The labor involved in the care of numerous
stoves is considerable, and hence the advent of a central heating
stove, or furnace, was a great saving in strength and fuel. A furnace
is a stove arranged as in Figure 13. The stove _S_, like all other
stoves, has an inlet for air and an outlet _C_ for smoke; but in
addition, it has built around it a chamber in which air circulates and
is warmed. The air warmed by the stove is forced upward by cold air
which enters from outside. For example, cold air constantly entering
at _E_ drives the air heated by _S_ through pipes and ducts to the
rooms to be heated.

The metal pipes which convey the heated air from the furnace to the
ducts are sometimes covered with felt, asbestos, or other
non-conducting material in order that heat may not be lost during
transmission. The ducts which receive the heated air from the pipes
are built in the non-conducting walls of the house, and hence lose
practically no heat. The air which reaches halls and rooms is
therefore warm, in spite of its long journey from the cellar.

[Illustration: FIG. 13.--A furnace. Pipes conduct hot air to the
rooms.]

Not only houses are warmed by a central heating stove, but whole
communities sometimes depend upon a central heating plant. In the
latter case, pipes closely wrapped with a non-conducting material
carry steam long distances underground to heat remote buildings.
Overbrook and Radnor, Pa., are towns in which such a system is used.

10. Hot-water Heating. The heated air which rises from furnaces is
seldom hot enough to warm large buildings well; hence furnace heating
is being largely supplanted by hot-water heating.

The principle of hot-water heating is shown by the following simple
experiment. Two flasks and two tubes are arranged as in Figure 15, the
upper flask containing a colored liquid and the lower flask clear
water. If heat is applied to _B_, one can see at the end of a few
seconds the downward circulation of the colored liquid and the upward
circulation of the clear water. If we represent a boiler by _B_, a
radiator by the coiled tube, and a safety tank by _C_, we shall have a
very fair illustration of the principle of a hot-water heating system.
The hot water in the radiators cools and, in cooling, gives up its
heat to the rooms and thus warms them.

[Illustration: FIG. 14.--Hot-water heating.]

In hot-water heating systems, fresh air is not brought to the rooms,
for the radiators are closed pipes containing hot water. It is largely
for this reason that thoughtful people are careful to raise windows at
intervals. Some systems of hot-water heating secure ventilation by
confining the radiators to the basement, to which cold air from
outside is constantly admitted in such a way that it circulates over
the radiators and becomes strongly heated. This warm fresh air then
passes through ordinary flues to the rooms above.

[Illustration: FIG. 15.--The principle of hot-water heating.]

In Figure 16, a radiator is shown in a boxlike structure in the
cellar. Fresh air from outside enters a flue at the right, passes the
radiator, where it is warmed, and then makes its way to the room
through a flue at the left. The warm air which thus enters the room is
thoroughly fresh. The actual labor involved in furnace heating and in
hot-water heating is practically the same, since coal must be fed to
the fire, and ashes must be removed; but the hot-water system has the
advantage of economy and cleanliness.

[Illustration: FIG. 16.--Fresh air from outside circulates over the
radiators and then rises into the rooms to be heated.]

11. Fresh Air. Fresh air is essential to normal healthy living, and
2000 cubic feet of air per hour is desirable for each individual. If a
gentle breeze is blowing, a barely perceptible opening of a window
will give the needed amount, even if there are no additional drafts of
fresh air into the room through cracks. Most houses are so loosely
constructed that fresh air enters imperceptibly in many ways, and
whether we will or no, we receive some fresh air. The supply is,
however, never sufficient in itself and should not be depended upon
alone. At night, or at any other time when gas lights are required,
the need for ventilation increases, because every gas light in a room
uses up the same amount of air as four people.

[Illustration: FIG. 17.--The air which goes to the schoolrooms is
warmed by passage over the radiators.]

In the preceding Section, we learned that many houses heated by hot
water are supplied with fresh-air pipes which admit fresh air into
separate rooms or into suites of rooms. In some cases the amount which
enters is so great that the air in a room is changed three or four
times an hour. The constant inflow of cold air and exit of warm air
necessitates larger radiators and more hot water and hence more coal
to heat the larger quantity of water, but the additional expense is
more than compensated by the gain in health.

12. Winds and Currents. The gentlest summer breezes and the fiercest
blasts of winter are produced by the unequal heating of air. We have
seen that the air nearest to a stove or hot object becomes hotter than
the adjacent air, that it tends to expand and is replaced and pushed
upward and outward by colder, heavier air falling downward. We have
learned also that the moving liquid or gas carries with it heat which
it gradually gives out to surrounding bodies.

When a liquid or a gas moves away from a hot object, carrying heat
with it, the process is called _convection_.

Convection is responsible for winds and ocean currents, for land and
sea breezes, and other daily phenomena.

The Gulf Stream illustrates the transference of heat by convection. A
large body of water is strongly heated at the equator, and then moves
away, carrying heat with it to distant regions, such as England and
Norway.

Owing to the shape of the earth and its position with respect to the
sun, different portions of the earth are unequally heated. In those
portions where the earth is greatly heated, the air likewise will be
heated; there will be a tendency for the air to rise, and for the cold
air from surrounding regions to rush in to fill its place. In this way
winds are produced. There are many circumstances which modify winds
and currents, and it is not always easy to explain their direction
and velocity, but one very definite cause is the unequal heating of
the surface of the earth.

13. Conduction. A poker used in stirring a fire becomes hot and
heats the hand grasping the poker, although only the opposite end of
the poker has actually been in the fire. Heat from the fire passed
into the poker, traveled along it, and warmed it. When heat flows in
this way from a warm part of a body to a colder part, the process is
called _conduction_. A flatiron is heated by conduction, the heat from
the warm stove passing into the cold flatiron and gradually heating
it.

In convection, air and water circulate freely, carrying heat with
them; in conduction, heat flows from a warm region toward a cold
region, but there is no apparent motion of any kind.

Heat travels more readily through some substances than through others.
All metals conduct heat well; irons placed on the fire become heated
throughout and cannot be grasped with the bare hand; iron utensils are
frequently made with wooden handles, because wood is a poor conductor
and does not allow heat from the iron to pass through it to the hand.
For the same reason a burning match may be held without discomfort
until the flame almost reaches the hand.

Stoves and radiators are made of metal, because metals conduct heat
readily, and as fast as heat is generated within the stove by the
burning of fuel, or introduced into the radiator by the hot water, the
heat is conducted through the metal and escapes into the room.

Hot-water pipes and steam pipes are usually wrapped with a
non-conducting substance, or insulator, such as asbestos, in order
that the heat may not escape, but shall be retained within the pipes
until it reaches the radiators within the rooms.

The invention of the "Fireless Cooker" depended in part upon the
principle of non-conduction. Two vessels, one inside the other, are
separated by sawdust, asbestos, or other poor conducting material
(Fig. 18). Foods are heated in the usual way to the boiling point or
to a high temperature, and are then placed in the inner vessel. The
heat of the food cannot escape through the non-conducting material
which surrounds it, and hence remains in the food and slowly cooks it.

[Illustration: FIG. 18.--A fireless cooker.]

A very interesting experiment for the testing of the efficacy of
non-conductors may be easily performed. Place hot water in a metal
vessel, and note by means of a thermometer the _rapidity_ with which
the water cools; then place water of the same temperature in a second
metal vessel similar to the first, but surrounded by asbestos or other
non-conducting material, and note the _slowness_ with which the
temperature falls.

Chemical Change, an Effect of Heat. This effect of heat has a vital
influence on our lives, because the changes which take place when food
is cooked are due to it. The doughy mass which goes into the oven,
comes out a light spongy loaf; the small indigestible rice grain comes
out the swollen, fluffy, digestible grain. Were it not for the
chemical changes brought about by heat, many of our present foods
would be useless to man. Hundreds of common materials like glass,
rubber, iron, aluminum, etc., are manufactured by processes which
involve chemical action caused by heat.




CHAPTER II

TEMPERATURE AND HEAT


14. Temperature not a Measure of the Amount of Heat Present. If two
similar basins containing unequal quantities of water are placed in
the sunshine on a summer day, the smaller quantity of water will
become quite warm in a short period of time, while the larger quantity
will become only lukewarm. Both vessels receive the same amount of
heat from the sun, but in one case the heat is utilized in heating to
a high temperature a small quantity of water, while in the second case
the heat is utilized in warming to a lower degree a larger quantity of
water. Equal amounts of heat do not necessarily produce equivalent
temperatures, and equal temperatures do not necessarily indicate equal
amounts of heat. It takes more heat to raise a gallon of water to the
boiling point than it does to raise a pint of water to the boiling
point, but a thermometer would register the same temperature in the
two cases. The temperature of boiling water is 100° C. whether there
is a pint of it or a gallon. Temperature is independent of the
quantity of matter present; but the amount of heat contained in a
substance at any temperature is not independent of quantity, being
greater in the larger quantity.

15. The Unit of Heat. It is necessary to have a unit of heat just as
we have a unit of length, or a unit of mass, or a unit of time. One
unit of heat is called a _calorie_, and is the amount of heat which
will change the temperature of 1 gram of water 1° C. It is the amount
of heat given out by 1 gram of water when its temperature falls 1° C.,
or the amount of heat absorbed by 1 gram of water when its temperature
rises 1° C. If 400 grams of water are heated from 0° to 5° C., the
amount of heat which has entered the water is equivalent to 5 × 400 or
2000 calories; if 200 grams of water cool from 25° to 20° C., the heat
given out by the water is equivalent to 5 × 200 or 1000 calories.

16. Some Substances Heat more readily than Others. If two equal
quantities of water at the same temperature are exposed to the sun for
the same length of time, their final temperatures will be the same.
If, however, equal quantities of different substances are exposed, the
temperatures resulting from the heating will not necessarily be the
same. If a basin containing 1 lb. of mercury is put on the fire, side
by side with a basin containing an equal quantity of water, the
temperatures of the two substances will vary greatly at the end of a
short time. The mercury will have a far higher temperature than the
water, in spite of the fact that the amount of mercury is as great as
the amount of water and that the heat received from the fire has been
the same in each case. Mercury is not so difficult to heat as water;
less heat being required to raise its temperature 1° than is required
to raise the temperature of an equal quantity of water 1°. In fact,
mercury is 30 times as easy to heat as water, and it requires only one
thirtieth as much fire to heat a given quantity of mercury 1° as to
heat the same quantity of water 1°.

17. Specific Heat. We know that different substances are differently
affected by heat. Some substances, like water, change their
temperature slowly when heated; others, like mercury, change their
temperature very rapidly when heated. The number of calories needed by
1 gram of a substance in order that its temperature may be increased
1° C. is called the _specific heat_ of a substance; or, specific heat
is the number of calories given out by 1 gram of a substance when its
temperature falls 1° C. For experiments on the determination of
specific heat, see Laboratory Manual.

Water has the highest specific heat of any known substance except
hydrogen; that is, it requires more heat to raise the temperature of
water a definite number of degrees than it does to raise the
temperature of an equal amount of any other substance the same number
of degrees. Practically this same thing can be stated in another way:
Water in cooling gives out more heat than any other substance in
cooling through the same number of degrees. For this reason water is
used in foot warmers and in hot-water bags. If a copper lid were used
as a foot warmer, it would give the feet only .095 as much heat as an
equal weight of water; a lead weight only .031 as much heat as water.
Flatirons are made of iron because of the relatively high specific
heat of iron. The flatiron heats slowly and cools slowly, and, because
of its high specific heat, not only supplies the laundress with
considerable heat, but eliminates for her the frequent changing of the
flatiron.

18. Water and Weather. About four times as much heat is required to
heat a given quantity of water one degree as to heat an equal quantity
of earth. In summer, when the rocks and the sand along the shore are
burning hot, the ocean and lakes are pleasantly cool, although the
amount of heat present in the water is as great as that present in the
earth. In winter, long after the rocks and sand have given out their
heat and have become cold, the water continues to give out the vast
store of heat accumulated during the summer. This explains why lands
situated on or near large bodies of water usually have less variation
in temperature than inland regions. In the summer the water cools the
region; in the winter, on the contrary, the water heats the region,
and hence extremes of temperature are practically unknown.

19. Sources of Heat. Most of the heat which we enjoy and use we owe
to the sun. The wood which blazes on the hearth, the coal which glows
in the furnace, and the oil which burns in the stove owe their
existence to the sun.

Without the warmth of the sun seeds could not sprout and develop into
the mighty trees which yield firewood. Even coal, which lies buried
thousands of feet below the earth's surface, owes its existence in
part to the sun. Coal is simply buried vegetation,--vegetation which
sprouted and grew under the influence of the sun's warm rays. Ages ago
trees and bushes grew "thick and fast," and the ground was always
covered with a deep layer of decaying vegetable matter. In time some
of this vast supply sank into the moist soil and became covered with
mud. Then rock formed, and the rock pressed down upon the sunken
vegetation. The constant pressure, the moisture in the ground, and
heat affected the underground vegetable mass, and slowly changed it
into coal.

The buried forest and thickets were not all changed into coal. Some
were changed into oil and gas. Decaying animal matter was often mixed
with the vegetable mass. When the mingled animal and vegetable matter
sank into moist earth and came under the influence of pressure, it was
slowly changed into oil and gas.

The heat of our bodies comes from the foods which we eat. Fruits,
grain, etc., could not grow without the warmth and the light of the
sun. The animals which supply our meats likewise depend upon the sun
for light and warmth.

The sun, therefore, is the great source of heat; whether it is the
heat which comes directly from the sun and warms the atmosphere, or
the heat which comes from burning coal, wood, and oil.




CHAPTER III

OTHER FACTS ABOUT HEAT


20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is
placed above a flame, the temperature of the water gradually
increases, and soon small bubbles form at the bottom of the kettle and
begin to rise through the water. At first the bubbles do not get far
in their ascent, but disappear before they reach the surface; later,
as the water gets hotter and hotter, the bubbles become larger and
more numerous, rise higher and higher, and finally reach the surface
and pass from the water into the air; steam comes from the vessel, and
the water is said to _boil_. The temperature at which a liquid boils
is called the boiling point.

While the water is heating, the temperature steadily rises, but as
soon as the water begins to boil the thermometer reading becomes
stationary and does not change, no matter how hard the water boils and
in spite of the fact that heat from the flame is constantly passing
into the water.

If the flame is removed from the boiling water for but a second, the
boiling ceases; if the flame is replaced, the boiling begins again
immediately. Unless heat is constantly supplied, water at the boiling
point cannot be transformed into steam.

_The number of calories which must be supplied to 1 gram of water at
the boiling point in order to change it into steam at the same
temperature is called the heat of vaporization_; it is the heat
necessary to change 1 gram of water at the boiling point into steam of
the same temperature.

21. The Amount of Heat Absorbed. The amount of heat which must be
constantly supplied to water at the boiling point in order to change
it into steam is far greater than we realize. If we put a beaker of
ice water (water at 0° C.) over a steady flame, and note (1) the time
which elapses before the water begins to boil, and (2) the time which
elapses before the boiling water completely boils away, we shall see
that it takes about 5-1/4 times as long to change water into steam as
it does to change its temperature from 0° C. to 100° C. Since, with a
steady flame, it takes 5-1/4 times as long to change water into steam
as it does to change its temperature from 0° C. to the boiling point,
we conclude that it takes 5-1/4 times as much heat to convert water at
the boiling point into steam as it does to raise it from the
temperature of ice water to that of boiling water.

The amount of heat necessary to raise the temperature of 1 gram of
water 1° C. is equal to 1 calorie, and the amount necessary to raise
the temperature 100° C. is equal to 100 calories; hence the amount of
heat necessary to convert 1 gram of water at the boiling point into
steam at that same temperature is equal to approximately 525 calories.
Very careful experiments show the exact heat of vaporization to be
536.1 calories. (See Laboratory Manual.)

22. General Truths. Statements similar to the above hold for other
liquids and for solutions. If milk is placed upon a stove, the
temperature rises steadily until the boiling point is reached; further
heating produces, not a change in temperature, but a change of the
water of the milk into steam. As soon as the milk, or any other liquid
food, comes to a boil, the gas flame should be lowered until only an
occasional bubble forms, because so long as any bubbles form the
temperature is that of the boiling point, and further heat merely
results in waste of fuel.

We find by experiment that every liquid has its own specific boiling
point; for example, alcohol boils at 78° C. and brine at 103° C. Both
specific heat and the heat of vaporization vary with the liquid used.

23. Condensation. If one holds a cold lid in the steam of boiling
water, drops of water gather on the lid; the steam is cooled by
contact with the cold lid and _condenses_ into water. Bottles of water
brought from a cold cellar into a warm room become covered with a mist
of fine drops of water, because the moisture in the air, chilled by
contact with the cold bottles, immediately condenses into drops of
water. Glasses filled with ice water show a similar mist.

In Section 21, we saw that 536 calories are required to change 1 gram
of water into steam; if, now, the steam in turn condenses into water,
it is natural to expect a release of the heat used in transforming
water into steam. Experiment shows not only that vapor gives out heat
during condensation, but that the amount of heat thus set free is
exactly equal to the amount absorbed during vaporization. (See
Laboratory Manual.)

We learn that the heat of vaporization is the same whether it is
considered as the heat absorbed by 1 gram of water in its change to
steam, or as the heat given out by 1 gram of steam during its
condensation into water.

24. Practical Application. We understand now the value of steam as a
heating agent. Water is heated in a boiler in the cellar, and the
steam passes through pipes which run to the various rooms; there the
steam condenses into water in the radiators, each gram of steam
setting free 536 calories of heat. When we consider the size of the
radiators and the large number of grams of steam which they contain,
and consider further that each gram in condensing sets free 536
calories, we understand the ease with which buildings are heated by
steam.

Most of us have at times profited by the heat of condensation. In cold
weather, when there is a roaring fire in the range, the water
frequently becomes so hot that it "steams" out of open faucets. If, at
such times, the hot water is turned on in a small cold bathroom, and
is allowed to run until the tub is well filled, vapor condenses on
windows, mirrors, and walls, and the cold room becomes perceptibly
warmer. The heat given out by the condensing steam passes into the
surrounding air and warms the room.

There is, however, another reason for the rise in temperature. If a
large pail of hot soup is placed in a larger pail of cold water, the
soup will gradually cool and the cold water will gradually become
warmer. A red-hot iron placed on a stand gradually cools, but warms
the stand. A hot body loses heat so long as a cooler body is near it;
the cold object is heated at the expense of the warmer object, and one
loses heat and the other gains heat until the temperature of both is
the same. Now the hot water in the tub gradually loses heat and the
cold air of the room gradually gains heat by convection, but the
amount given the room by convection is relatively small compared with
the large amount set free by the condensing steam.

25. Distillation. If impure, muddy water is boiled, drops of water
will collect on a cold plate held in the path of the steam, but the
drops will be clear and pure. When impure water is boiled, the steam
from it does not contain any of the impurities because these are left
behind in the vessel. If all the water were allowed to boil away, a
layer of mud or of other impurities would be found at the bottom of
the vessel. Because of this fact, it is possible to purify water in a
very simple way. Place over a fire a large kettle closed except for a
spout which is long enough to reach across the stove and dip into a
bottle. As the liquid boils, steam escapes through the spout, and on
reaching the cold bottle condenses and drops into the bottle as pure
water. The impurities remain behind in the kettle. Water freed from
impurities in this way is called _distilled water_, and the process is
called _distillation_ (Fig. 19). By this method, the salt water of the
ocean may be separated into pure drinking water and salt, and many of
the large ocean liners distill from the briny deep all the drinking
water used on their ocean voyages.

[Illustration: FIG. 19.--In order that the steam which passes through
the coiled tube may be quickly cooled and condensed, cold water is
made to circulate around the coil. The condensed steam escapes at
_w_.]

Commercially, distillation is a very important process. Turpentine,
for example, is made by distilling the sap of pine trees. Incisions
are cut in the bark of the long-leaf pine trees, and these serve as
channels for the escape of crude resin. This crude liquid is collected
in barrels and taken to a distillery, where it is distilled into
turpentine and rosin. The turpentine is the product which passes off
as vapor, and the rosin is the mass left in the boiler after the
distillation of the turpentine.

26. Evaporation. If a stopper is left off a cologne bottle, the
contents of the bottle will slowly evaporate; if a dish of water is
placed out of doors on a hot day, evaporation occurs very rapidly. The
liquids which have disappeared from the bottle and the dish have
passed into the surrounding air in the form of vapor. In Section 20,
we saw that water could not pass into vapor without the addition of
heat; now the heat necessary for the evaporation of the cologne and
water was taken from the air, leaving it slightly cooler. If wet hands
are not dried with a towel, but are left to dry by evaporation, heat
is taken from the hand in the process, leaving a sensation of
coolness. Damp clothing should never be worn, because the moisture in
it tends to evaporate at the expense of the bodily heat, and this
undue loss of heat from the body produces chills. After a bath the
body should be well rubbed, otherwise evaporation occurs at the
expense of heat which the body cannot ordinarily afford to lose.

Evaporation is a slow process occurring at all times; it is hastened
during the summer, because of the large amount of heat present in the
atmosphere. Many large cities make use of the cooling effect of
evaporation to lower the temperature of the air in summer; streets are
sprinkled not only to lay the dust, but in order that the surrounding
air may be cooled by the evaporation of the water.

Some thrifty housewives economize by utilizing the cooling effects of
evaporation. Butter, cheese, and other foods sensitive to heat are
placed in porous vessels wrapped in wet cloths. Rapid evaporation of
the water from the wet cloths keeps the contents of the jars cool, and
that without expense other than the muscular energy needed for wetting
the cloths frequently.

27. Rain, Snow, Frost, Dew. The heat of the sun causes constant
evaporation of the waters of oceans, rivers, streams, and marshes, and
the water vapor set free by evaporation passes into the air, which
becomes charged with vapor or is said to be humid. Constant, unceasing
evaporation of our lakes, streams, and pools would mean a steady
decrease in the supply of water available for daily use, if the
escaped water were all retained by the atmosphere and lost to the
earth. But although the escaped vapor mingles with the atmosphere,
hovering near the earth's surface, or rising far above the level of
the mountains, it does not remain there permanently. When this vapor
meets a cold wind or is chilled in any way, condensation takes place,
and a mass of tiny drops of water or of small particles of snow is
formed. When these drops or particles become large enough, they fall
to the earth as rain or snow, and in this way the earth is compensated
for the great loss of moisture due to evaporation. Fog is formed when
vapor condenses near the surface of the earth, and when the drops are
so small that they do not fall but hover in the air, the fog is said
"not to lift" or "not to clear."

If ice water is poured into a glass, a mist will form on the outside
of the glass. This is because the water vapor in the air becomes
chilled by contact with the glass and condenses. Often leaves and
grass and sidewalks are so cold that the water vapor in the atmosphere
condenses on them, and we say a heavy dew has formed. If the
temperature of the air falls to the freezing point while the dew is
forming, the vapor is frozen and frost is seen instead of dew.

The daily evaporation of moisture into the atmosphere keeps the
atmosphere more or less full of water vapor; but the atmosphere can
hold only a definite amount of vapor at a given temperature, and as
soon as it contains the maximum amount for that temperature, further
evaporation ceases. If clothes are hung out on a damp, murky day they
do not dry, because the air contains all the moisture it can hold, and
the moisture in the clothes has no chance to evaporate. When the air
contains all the moisture it can hold, it is said to be saturated, and
if a slight fall in temperature occurs when the air is saturated,
condensation immediately begins in the form of rain, snow, or fog. If,
however, the air is not saturated, a fall in temperature may occur
without producing precipitation. The temperature at which air is
saturated and condensation begins is called the _dew point_.

28. How Chills are Caused. The discomfort we feel in an overcrowded
room is partly due to an excess of moisture in the air, resulting from
the breathing and perspiration of many persons. The air soon becomes
saturated with vapor and cannot take away the perspiration from our
bodies, and our clothing becomes moist and our skin tender. When we
leave the crowded "tea" or lecture and pass into the colder, drier,
outside air, clothes and skin give up their load of moisture through
sudden evaporation. But evaporation requires heat, and this heat is
taken from our bodies, and a chill results.

Proper ventilation would eliminate much of the physical danger of
social events; fresh, dry air should be constantly admitted to crowded
rooms in order to replace the air saturated by the breath and
perspiration of the occupants.

29. Weather Forecasts. When the air is near the saturation point,
the weather is oppressive and is said to be very humid. For comfort
and health, the air should be about two thirds saturated. The presence
of some water vapor in the air is absolutely necessary to animal and
plant life. In desert regions where vapor is scarce the air is so dry
that throat trouble accompanied by disagreeable tickling is prevalent;
fallen leaves become so dry that they crumble to dust; plants lose
their freshness and beauty.

The likelihood of rain or frost is often determined by temperature and
humidity. If the air is near saturation and the temperature is
falling, it is safe to predict bad weather, because the fall of
temperature will probably cause rapid condensation, and hence rain.
If, however, the air is not near the saturation point, a fall in
temperature will not necessarily produce bad weather.

The measurement of humidity is of far wider importance than the mere
forecasting of local weather conditions. The close relation between
humidity and health has led many institutions, such as hospitals,
schools, and factories, to regulate the humidity of the atmosphere as
carefully as they do the temperature. Too great humidity is
enervating, and not conducive to either mental or physical exertion;
on the other hand, too dry air is equally harmful. In summer the
humidity conditions cannot be well regulated, but in winter, when
houses are artificially heated, the humidity of a room can be
increased by placing pans of water near the registers or on radiators.

30. Heat Needed to Melt Substances. If a spoon is placed in a vessel
of hot water for a few seconds and then removed, it will be warmer
than before it was placed in the hot water. If a lump of melting ice
is placed in the vessel of hot water and then removed, the ice will
not be warmer than before, but there will be less of it. The heat of
the water has been used in melting the ice, not in changing its
temperature.

If, on a bitter cold day, a pail of snow is brought into a warm room
and a thermometer is placed in the snow, the temperature rises
gradually until 32° F. is reached, when it becomes stationary, and the
snow begins to melt. If the pail is put on the fire, the temperature
still remains 32°F., but the snow melts more rapidly. As soon as all
the snow is completely melted, however, the temperature begins to rise
and rises steadily until the water boils, when it again becomes
stationary and remains so during the passage of water into vapor.

We see that heat must be supplied to ice at 0° C. or 32° F. in order
to change it into water, and further, that the temperature of the
mixture does not rise so long as any ice is present, no matter how
much heat is supplied. The amount of heat necessary to melt 1 gram of
ice is easily calculated. (See Laboratory Manual.)

Heat must be supplied to ice to melt it. On the other hand, water, in
freezing, loses heat, and the amount of heat lost by freezing water is
exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is
called the _heat of fusion_ of water.

31. Climate. Water, in freezing, loses heat, even though its
temperature remains at 0° C. Because water loses heat when it freezes,
the presence of large streams of water greatly influences the climate
of a region. In winter the heat from the freezing water keeps the
temperature of the surrounding higher than it would naturally be, and
consequently the cold weather is less severe. In summer water
evaporates, heat is taken from the air, and consequently the warm
weather is less intense.

32. Molding of Glass and Forging of Iron. The fire which is hot
enough to melt a lump of ice may not be hot enough to melt an iron
poker; on the other hand, it may be sufficiently hot to melt a tin
spoon. Different substances melt, or liquefy, at different
temperatures; for example, ice melts at 0° C., and tin at 233° C.,
while iron requires the relatively high temperature of 1200° C. Most
substances have a definite melting or freezing point which never
changes so long as the surrounding conditions remain the same.

But while most substances have a definite melting point, some
substances do not. If a glass rod is held in a Bunsen burner, it will
gradually grow softer and softer, and finally a drop of molten glass
will fall from the end of the rod into the fire. The glass did not
suddenly become a liquid at a definite temperature; instead it
softened gradually, and then melted. While glass is in the soft,
yielding, pliable state, it is molded into dishes, bottles, and other
useful objects, such as lamp shades, globes, etc. (Fig. 20). If glass
melted at a definite temperature, it could not be molded in this way.
Iron acts in a similar manner, and because of this property the
blacksmith can shape his horseshoes, and the workman can make his
engines and other articles of daily service to man.

[Illustration: FIG. 20.--Molten glass being rolled into a form
suitable for window panes.]

33. Strange Behavior of Water. One has but to remember that bottles
of water burst when they freeze, and that ice floats on water like
wood, to know that water expands on freezing or on solidifying. A
quantity of water which occupies 100 cubic feet of space will, on
becoming ice, need 109 cubic feet of space. On a cold winter night the
water sometimes freezes in the water pipes, and the pipes burst. Water
is very peculiar in expanding on solidification, because most
substances contract on solidifying; gelatin and jelly, for example,
contract so much that they shrink from the sides of the dish which
contains them.

If water contracted in freezing, ice would be heavier than water and
would sink in ponds and lakes as fast as it formed, and our streams
and ponds would become masses of solid ice, killing all animal and
plant life. But the ice is lighter than water and floats on top, and
animals in the water beneath are as free to live and swim as they were
in the warm sunny days of summer. The most severe winter cannot freeze
a deep lake solid, and in the coldest weather a hole made in the ice
will show water beneath the surface. Our ice boats cut and break the
ice of the river, and through the water beneath our boats daily ply
their way to and fro, independent of winter and its blighting blasts.

While most of us are familiar with the bursting of water pipes on a
cold night, few of us realize the influence which freezing water
exerts on the character of the land around us.

Water sinks into the ground and, on the approach of winter, freezes,
expanding about one tenth of its volume; the expanding ice pushes the
earth aside, the force in some cases being sufficient to dislodge even
huge rocks. In the early days in New England it was said by the
farmers that "rocks grew," because fields cleared of stones in the
fall became rock covered with the approach of spring; the rocks and
stones hidden underground and unseen in the fall were forced to the
surface by the winter's expansion. We have all seen fence posts and
bricks pushed out of place because of the heaving of the soil beneath
them. Often householders must relay their pavements and walks because
of the damage done by freezing water.

The most conspicuous effect of the expansive power of freezing water
is seen in rocky or mountainous regions (Fig. 21). Water easily finds
entrance into the cracks and crevices of the rocks, where it lodges
until frozen; then it expands and acts like a wedge, widening cracks,
chiseling off edges, and even breaking rocks asunder. In regions where
frequent frosts occur, the destructive action of water works constant
changes in the appearance of the land; small cracks and crevices are
enlarged, massive rocks are pried up out of position, huge slabs are
split off, and particles large and small are forced from the parent
rock. The greater part of the debris and rubbish brought down from the
mountain slopes by the spring rains owes its origin to the fact that
water expands when it freezes.

[Illustration: FIG. 21.--The destruction caused by freezing water.]

34. Heat Necessary to Dissolve a Substance. It requires heat to
dissolve any substance, just as it requires heat to change ice to
water. If a handful of common salt is placed in a small cup of water
and stirred with a thermometer, the temperature of the mixture falls
several degrees. This is just what one would expect, because the heat
needed to liquefy the salt must come from somewhere, and naturally it
comes from the water, thereby lowering the temperature of the water.
We know very well that potatoes cease boiling if a pinch of salt is
put in the water; this is because the temperature of the water has
been lowered by the amount of heat necessary to dissolve the salt.

Let some snow or chopped ice be placed in a vessel and mixed with one
third its weight of coarse salt; if then a small tube of cold water is
placed in this mixture, the water in the test tube will soon freeze
solid. As soon as the snow and salt are mixed they melt. The heat
necessary for this comes in part from the air and in part from the
water in the test tube, and the water in the tube becomes in
consequence cold enough to freeze. But the salt mixture does not
freeze because its freezing point is far below that of pure water. The
use of salt and ice in ice-cream freezers is a practical application
of this principle. The heat necessary for melting the mixture of salt
and ice is taken from the cream which thus becomes cold enough to
freeze.




CHAPTER IV

BURNING OR OXIDATION


35. Why Things Burn. The heat of our bodies comes from the food we
eat; the heat for cooking and for warming our houses comes from coal.
The production of heat through the burning of coal, or oil, or gas, or
wood, is called combustion. Combustion cannot occur without the
presence of a substance called oxygen, which exists rather abundantly
in the air; that is, one fifth of our atmosphere consists of this
substance which we call oxygen. We throw open our windows to allow
fresh air to enter, and we take walks in order to breathe the pure air
into our lungs. What we need for the energy and warmth of our bodies
is the oxygen in the air. Whether we burn gas or wood or coal, the
heat which is produced comes from the power which these various
substances possess to combine with oxygen. We open the draft of a
stove that it may "draw well": that it may secure oxygen for burning.
We throw a blanket over burning material to smother the fire: to keep
oxygen away from it. Burning, or oxidation, is combining with oxygen,
and the more oxygen you add to a fire, the hotter the fire will burn,
and the faster. The effect of oxygen on combustion may be clearly seen
by thrusting a smoldering splinter into a jar containing oxygen; the
smoldering splinter will instantly flare and blaze, while if it is
removed from the jar, it loses its flame and again burns quietly.
Oxygen for this experiment can be produced in the following way.

[Illustration: FIG. 22.--Preparing oxygen from potassium chlorate and
manganese dioxide.]

36. How to Prepare Oxygen. Mix a small quantity of potassium
chlorate with an equal amount of manganese dioxide and place the
mixture in a strong test tube. Close the mouth of the tube with a
one-hole rubber stopper in which is fitted a long, narrow tube, and
clamp the test tube to an iron support, as shown in Figure 22. Fill
the trough with water until the shelf is just covered and allow the
end of the delivery tube to rest just beneath the hole in the shelf.
Fill a medium-sized bottle with water, cover it with a glass plate,
invert the bottle in the trough, and then remove the glass plate. Heat
the test tube very gently, and when gas bubbles out of the tube, slip
the bottle over the opening in the shelf, so that the tube runs into
the bottle. The gas will force out the water and will finally fill the
bottle. When all the water has been forced out, slip the glass plate
under the mouth of the bottle and remove the bottle from the trough.
The gas in the bottle is oxygen.

Everywhere in a large city or in a small village, smoke is seen,
indicating the presence of fire; hence there must exist a large supply
of oxygen to keep all the fires alive. The supply of oxygen needed
for the fires of the world comes largely from the atmosphere.

37. Matches. The burning material is ordinarily set on fire by
matches, thin strips of wood tipped with sulphur or phosphorus, or
both. Phosphorus can unite with oxygen at a fairly low temperature,
and if phosphorus is rubbed against a rough surface, the friction
produced will raise the temperature of the phosphorus to a point where
it can combine with oxygen. The burning phosphorus kindles the wood of
the match, and from the burning match the fire is kindled. If you want
to convince yourself that friction produces heat, rub a cent
vigorously against your coat and note that the cent becomes warm.
Matches have been in use less than a hundred years. Primitive man
kindled his camp fire by rubbing pieces of dry wood together until
they took fire, and this method is said to be used among some isolated
distant tribes at the present time. A later and easier way was to
strike flint and steel together and to catch the spark thus produced
on tinder or dry fungus. Within the memory of some persons now living,
the tinder box was a valuable asset to the home, particularly in the
pioneer regions of the West.

38. Safety Matches. Ordinary phosphorus, while excellent as a
fire-producing material, is dangerously poisonous, and those to whom
the dipping of wooden strips into phosphorus is a daily occupation
suffer with a terrible disease which usually attacks the teeth and
bones of the jaw. The teeth rot and fall out, abscesses form, and
bones and flesh begin to decay; the only way to prevent the spread of
the disease is to remove the affected bone, and in some instances it
has been necessary to remove the entire jaw. Then, too, matches made
of yellow or white phosphorus ignite easily, and, when rubbed against
any rough surface, are apt to take fire. Many destructive fires have
been started by the accidental friction of such matches against rough
surfaces.

For these reasons the introduction of the so-called safety match was
an important event. When common phosphorus, in the dangerous and
easily ignited form, is heated in a closed vessel to about 250° C., it
gradually changes to a harmless red mass. The red phosphorus is not
only harmless, but it is difficult to ignite, and, in order to be
ignited by friction, must be rubbed on a surface rich in oxygen. The
head of a safety match is coated with a mixture of glue and
oxygen-containing compounds; the surface on which the match is to be
rubbed is coated with a mixture of red phosphorus and glue, to which
finely powdered glass is sometimes added in order to increase the
friction. Unless the head of the match is rubbed on the prepared
phosphorus coating, ignition does not occur, and accidental fires are
avoided.

Various kinds of safety matches have been manufactured in the last few
years, but they are somewhat more expensive than the ordinary form,
and hence manufacturers are reluctant to substitute them for the
cheaper matches. Some foreign countries, such as Switzerland, prohibit
the sale of the dangerous type, and it is hoped that the United States
will soon follow the lead of these countries in demanding the sale of
safety matches only.

39. Some Unfamiliar Forms of Burning. While most of us think of
burning as a process in which flames and smoke occur, there are in
reality many modes of burning accompanied by neither flame nor smoke.
Iron, for example, burns when it rusts, because it slowly combines
with the oxygen of the air and is transformed into new substances.
When the air is dry, iron does not unite with oxygen, but when
moisture is present in the air, the iron unites with the oxygen and
turns into iron rust. The burning is slow and unaccompanied by the
fire and smoke so familiar to us, but the process is none the less
burning, or combination with oxygen. Burning which is not accompanied
by any of the appearances of ordinary burning is known as oxidation.

The tendency of iron to rust lessens its efficiency and value, and
many devices have been introduced to prevent rusting. A coating of
paint or varnish is sometimes applied to iron in order to prevent
contact with air. The galvanizing of iron is another attempt to secure
the same result; in this process iron is dipped into molten zinc,
thereby acquiring a coating of zinc, and forming what is known as
galvanized iron. Zinc does not combine with oxygen under ordinary
circumstances, and hence galvanized iron is immune from rust.

Decay is a process of oxidation; the tree which rots slowly away is
undergoing oxidation, and the result of the slow burning is the
decomposed matter which we see and the invisible gases which pass into
the atmosphere. The log which blazes on our hearth gives out
sufficient heat to warm us; the log which decays in the forest gives
out an equivalent amount of heat, but the heat is evolved so slowly
that we are not conscious of it. Burning accompanied by a blaze and
intense heat is a rapid process; burning unaccompanied by fire and
appreciable heat is a slow, gradual process, requiring days, weeks,
and even long years for its completion.

Another form of oxidation occurs daily in the human body. In Section
35 we saw that the human body is an engine whose fuel is food; the
burning of that food in the body furnishes the heat necessary for
bodily warmth and the energy required for thought and action. Oxygen
is essential to burning, and the food fires within the body are kept
alive by the oxygen taken into the body at every breath by the lungs.
We see now one reason for an abundance of fresh air in daily life.

40. How to Breathe. Air, which is essential to life and health,
should enter the body through the nose and _not through the mouth_.
The peculiar nature and arrangement of the membranes of the nose
enable the nostrils to clean, and warm, and moisten the air which
passes through them to the lungs. Floating around in the atmosphere
are dust particles which ought not to get into the lungs. The nose is
provided with small hairs and a moist inner membrane which serve as
filters in removing solid particles from the air, and in thus
purifying it before its entrance into the lungs.

In the immediate neighborhood of three Philadelphia high schools,
having an approximate enrollment of over 8000 pupils, is a huge
manufacturing plant which day and night pours forth grimy smoke and
soot into the atmosphere which must supply oxygen to this vast group
of young lives. If the vital importance of nose breathing is impressed
upon these young people, the harmful effect of the foul air may be
greatly lessened, the smoke particles and germs being held back by the
nose filters and never reaching the lungs. If, however, this principle
of hygiene is not brought to their attention, the dangerous habit of
breathing through the open, or at least partially open, mouth will
continue, and objectionable matter will pass through the mouth and
find a lodging place in the lungs.

There is another very important reason why nose breathing is
preferable to mouth breathing. The temperature of the human body is
approximately 98° F., and the air which enters the lungs should not be
far below this temperature. If air reaches the lungs through the nose,
its journey is relatively long and slow, and there is opportunity for
it to be warmed before it reaches the lungs. If, on the other hand,
air passes to the lungs by way of the mouth, the warming process is
brief and insufficient, and the lungs suffer in consequence.
Naturally, the gravest danger is in winter.

41. Cause of Mouth Breathing. Some people find it difficult to
breathe through the nostrils on account of growths, called adenoids,
in the nose. If you have a tendency toward mouth breathing, let a
physician examine your nose and throat.

Adenoids not only obstruct breathing and weaken the whole system
through lack of adequate air, but they also press upon the blood
vessels and nerves of the head and interfere with normal brain
development. Moreover, they interfere in many cases with the hearing,
and in general hinder activity and growth. The removal of adenoids is
simple, and carries with it only temporary pain and no danger. Some
physicians claim that the growths disappear in later years, but even
if that is true, the physical and mental development of earlier years
is lost, and the person is backward in the struggle for life and
achievement.

[Illustration: FIG. 23.--Intelligent expression is often lacking in
children with adenoid growths.]

42. How to Build a Fire. Substances differ greatly as to the ease
with which they may be made to burn or, in technical terms, with which
they may be made to unite with oxygen. For this reason, we put light
materials, like shavings, chips, and paper, on the grate, twisting the
latter and arranging it so that air (oxygen in the air) can reach a
large surface; upon this we place small sticks of wood, piling them
across each other so as to allow entrance for the oxygen; and finally
upon this we place our hard wood or coal.

The coal and the large sticks cannot be kindled with a match, but the
paper and shavings can, and these in burning will heat the large
sticks until they take fire and in turn kindle the coal.

43. Spontaneous Combustion. We often hear of fires "starting
themselves," and sometimes the statement is true. If a pile of oily
rags is allowed to stand for a time, the oily matter will begin to
combine slowly with oxygen and as a result will give off heat. The
heat thus given off is at first insufficient to kindle a fire; but as
the heat is retained and accumulated, the temperature rises, and
finally the kindling point is reached and the whole mass bursts into
flames. For safety's sake, all oily cloths should be burned or kept in
metal vessels.

44. The Treatment of Burns. In spite of great caution, burns from
fires, steam, or hot water do sometimes occur, and it is well to know
how to relieve the suffering caused by them and how to treat the
injury in order to insure rapid healing.

Burns are dangerous because they destroy skin and thus open up an
entrance into the body for disease germs, and in addition because they
lay bare nerve tissue which thereby becomes irritated and causes a
shock to the entire system.

In mild burns, where the skin is not broken but is merely reddened, an
application of moist baking soda brings immediate relief. If this
substance is not available, flour paste, lard, sweet oil, or vaseline
may be used.

In more severe burns, where blisters are formed, the blisters should
be punctured with a sharp, sterilized needle and allowed to discharge
their watery contents before the above remedies are applied.

In burns severe enough to destroy the skin, disinfection of the open
wound with weak carbolic acid or hydrogen peroxide is very necessary.
After this has been done, a soft cloth soaked in a solution of linseed
oil and limewater should be applied and the whole bandaged. In such a
case, it is important not to use cotton batting, since this sticks to
the rough surface and causes pain when removed.

45. Carbon Dioxide. _A Product of Burning._ When any fuel, such as
coal, gas, oil, or wood, burns, it sends forth gases into the
surrounding atmosphere. These gases, like air, are invisible, and were
unknown to us for a long time. The chief gas formed by a burning
substance is called carbon dioxide (CO_2) because it is composed of
one part of carbon and two parts of oxygen. This gas has the
distinction of being the most widely distributed gaseous compound of
the entire world; it is found in the ocean depths and on the mountain
heights, in brilliantly lighted rooms, and most abundantly in
manufacturing towns where factory chimneys constantly pour forth hot
gases and smoke.

Wood and coal, and in fact all animal and vegetable matter, contain
carbon, and when these substances burn or decay, the carbon in them
unites with oxygen and forms carbon dioxide.

The food which we eat is either animal or vegetable, and it is made
ready for bodily use by a slow process of burning within the body;
carbon dioxide accompanies this bodily burning of food just as it
accompanies the fires with which we are more familiar. The carbon
dioxide thus produced within the body escapes into the atmosphere with
the breath.

We see that the source of carbon dioxide is practically inexhaustible,
coming as it does from every stove, furnace, and candle, and further
with every breath of a living organism.

46. Danger of Carbon Dioxide. When carbon dioxide occurs in large
quantities, it is dangerous to health, because it interferes with
normal breathing, lessening the escape of waste matter through the
breath and preventing the access to the lungs of the oxygen necessary
for life. Carbon dioxide is not poisonous, but it cuts off the supply
of oxygen, just as water cuts it off from a drowning man.

Since every man, woman, and child constantly breathes forth carbon
dioxide, the danger in overcrowded rooms is great, and proper
ventilation is of vital importance.

47. Ventilation. In estimating the quantity of air necessary to keep
a room well aired, we must take into account the number of lights
(electric lights do not count) to be used, and the number of people to
occupy the room. The average house should provide at the _minimum_ 600
cubic feet of space for each person, and in addition, arrangements for
allowing at least 300 cubic feet of fresh air per person to enter
every hour.

In houses which have not a ventilating system, the air should be kept
fresh by intelligent action in the opening of doors and windows; and
since relatively few houses are equipped with a satisfactory system,
the following suggestions relative to intelligent ventilation are
offered.

1. Avoid drafts in ventilation.

2. Ventilate on the sheltered side of the house. If the wind is
blowing from the north, open south windows.

48. What Becomes of the Carbon Dioxide. When we reflect that carbon
dioxide is constantly being supplied to the atmosphere and that it is
injurious to health, the question naturally arises as to how the air
remains free enough of the gas to support life. This is largely
because carbon dioxide is an essential food of plants. Through their
leaves plants absorb it from the atmosphere, and by a wonderful
process break it up into its component parts, oxygen and carbon. They
reject the oxygen, which passes back to the air, but they retain the
carbon, which becomes a part of the plant structure. Plants thus serve
to keep the atmosphere free from an excess of carbon dioxide and, in
addition, furnish oxygen to the atmosphere.

[Illustration: FIG. 24.--Making carbon dioxide from marble and
hydrochloric acid.]

49. How to Obtain Carbon Dioxide. There are several ways in which
carbon dioxide can be produced commercially, but for laboratory use
the simplest is to mix in a test tube powdered marble, or chalk, and
hydrochloric acid, and to collect the effervescing gas as shown in
Figure 24. The substance which remains in the test tube after the gas
has passed off is a solution of a salt and water. From a mixture of
hydrochloric acid (HCl) and marble are obtained a salt, water, and
carbon dioxide, the desired gas.

50. A Commercial Use of Carbon Dioxide. If a lighted splinter is
thrust into a test tube containing carbon dioxide, it is promptly
extinguished, because carbon dioxide cannot support combustion; if a
stream of carbon dioxide and water falls upon a fire, it acts like a
blanket, covering the flames and extinguishing them. The value of a
fire extinguisher depends upon the amount of carbon dioxide and water
which it can furnish. A fire extinguisher is a metal case containing a
solution of bicarbonate of soda, and a glass vessel full of strong
sulphuric acid. As long as the extinguisher is in an upright position,
these substances are kept separate, but when the extinguisher is
inverted, the acid escapes from the bottle, and mixes with the soda
solution. The mingling liquids interact and liberate carbon dioxide.
A part of the gas thus liberated dissolves in the water of the soda
solution and escapes from the tube with the outflowing liquid, while a
portion remains undissolved and escapes as a stream of gas. The fire
extinguisher is therefore the source of a liquid containing the
fire-extinguishing substance and further the source of a stream of
carbon dioxide gas.

[Illustration: FIG. 25.--Inside view of a fire extinguisher.]

51. Carbon. Although carbon dioxide is very injurious to health,
both of the substances of which it is composed are necessary to life.
We ourselves, our bones and flesh in particular, are partly carbon,
and every animal, no matter how small or insignificant, contains some
carbon; while the plants around us, the trees, the grass, the flowers,
contain a by no means meager quantity of carbon.

Carbon plays an important and varied role in our life, and, in some
one of its many forms, enters into the composition of most of the
substances which are of service and value to man. The food we eat, the
clothes we wear, the wood and coal we burn, the marble we employ in
building, the indispensable soap, and the ornamental diamond, all
contain carbon in some form.

52. Charcoal. One of the most valuable forms of carbon is charcoal;
valuable not in the sense that it costs hundreds of dollars, but in
the more vital sense, that its use adds to the cleanliness, comfort,
and health of man.

The foul, bad-smelling gases which arise from sewers can be prevented
from escaping and passing to streets and buildings by placing charcoal
filters at the sewer exits. Charcoal is porous and absorbs foul gases,
and thus keeps the region surrounding sewers sweet and clean and free
of odor. Good housekeepers drop small bits of charcoal into vases of
flowers to prevent discoloration of the water and the odor of decaying
stems.

If impure water filters through charcoal, it emerges pure, having left
its impurities in the pores of the charcoal. Practically all household
filters of drinking water are made of charcoal. But such a device may
be a source of disease instead of a prevention of disease, unless the
filter is regularly cleaned or renewed. This is because the pores soon
become clogged with the impurities, and unless they are cleaned, the
water which flows through the filter passes through a bed of
impurities and becomes contaminated rather than purified. Frequent
cleansing or renewal of the filter removes this difficulty.

Commercially, charcoal is used on a large scale in the refining of
sugars, sirups, and oils. Sugar, whether it comes from the maple tree,
or the sugar cane, or the beet, is dark colored. It is whitened by
passage through filters of finely pulverized charcoal. Cider and
vinegar are likewise cleared by passage through charcoal.

The value of carbon, in the form of charcoal, as a purifier is very
great, whether we consider it a deodorizer, as in the case of the
sewage, or a decolorizer, as in the case of the refineries, or whether
we consider the service it has rendered man in the elimination of
danger from drinking water.

53. How Charcoal is Made. Charcoal may be made by heating wood in an
oven to which air does not have free access. The absence of air
prevents ordinary combustion, nevertheless the intense heat affects
the wood and changes it into new substances, one of which is charcoal.

The wood which smolders on the hearth and in the stove is charcoal in
the making. Formerly wood was piled in heaps, covered with sod or sand
to prevent access of oxygen, and then was set fire to; the smoldering
wood, cut off from an adequate supply of air, was slowly transformed
into charcoal. Scattered over the country one still finds isolated
charcoal kilns, crude earthen receptacles, in which wood thus deprived
of air was allowed to smolder and form charcoal. To-day charcoal is
made commercially by piling wood on steel cars and then pushing the
cars into strong walled chambers. The chambers are closed to prevent
access of air, and heated to a high temperature. The intense heat
transforms the wood into charcoal in a few hours. A student can make
in the laboratory sufficient charcoal for art lessons by heating in an
earthen vessel wood buried in sand. The process will be slow, however,
because the heat furnished by a Bunsen burner is not great, and the
wood is transformed slowly.

A form of charcoal known as animal charcoal, or bone black, is
obtained from the charred remains of animals rather than plants, and
may be prepared by burning bones and animal refuse as in the case of
the wood.

Destructive Distillation. When wood is burned without sufficient
air, it is changed into soft brittle charcoal, which is very different
from wood. It weighs only one fourth as much as the original wood. It
is evident that much matter must leave the wood during the process of
charcoal making. We can prove this by putting some dry shavings in a
strong test tube fitted with a delivery tube. When the wood is heated
a gas passes off which we may collect and burn. Other substances also
come off in gaseous form, but they condense in the water. Among these
are wood alcohol, wood tar, and acetic acid. In the older method of
charcoal making all these products were lost. Can you give any uses of
these substances?

54. Matter and Energy. When wood is burned, a small pile of ashes is
left, and we think of the bulk of the wood as destroyed. It is true we
have less matter that is available for use or that is visible to
sight, but, nevertheless, no matter has been destroyed. The matter of
which the wood is composed has merely changed its character, some of
it is in the condition of ashes, and some in the condition of
invisible gases, such as carbon dioxide, but none of it has been
destroyed. It is a principle of science that matter can neither be
destroyed nor created; it can only be changed, or transformed, and it
is our business to see that we do not heedlessly transform it into
substances which are valueless to us and our descendants; as, for
example, when our magnificent forests are recklessly wasted. The
smoke, gases, and ashes left in the path of a raging forest fire are
no compensation to us for the valuable timber destroyed. The sum total
of matter has not been changed, but the amount of matter which man can
use has been greatly lessened.

The principle just stated embodies one of the fundamental laws of
science, called the law of the _conservation of matter_.

A similar law holds for energy as well. We can transform electric
energy into the motion of trolley cars, or we can make use of the
energy of streams to turn the wheels of our mills, but in all these
cases we are transforming, not creating, energy.

When a ball is fired from a rifle, most of the energy of the gunpowder
is utilized in motion, but some is dissipated in producing a flash and
a report, and in heat. The energy of the gunpowder has been scattered,
but the sum of the various forms of energy is equal to the energy
originally stored away in the powder. The better the gun is, the less
will be the energy dissipated in smoke and heat and noise.




CHAPTER V

FOOD


55. The Body as a Machine. Wholesome food and fresh air are
necessary for a healthy body. Many housewives, through ignorance,
supply to their hard-working husbands and their growing sons and
daughters food which satisfies the appetite, but which does not give
to the body the elements needed for daily work and growth. Some foods,
such as lettuce, cucumbers, and watermelons, make proper and
satisfactory changes in diet, but are not strength giving. Other
foods, like peas and beans, not only satisfy the appetite, but supply
to the body abundant nourishment. Many immigrants live cheaply and
well with beans and bread as their main diet.

It is of vital importance that the relative value of different foods
as heat producers be known definitely; and just as the yard measures
length and the pound measures weight the calorie is used to measure
the amount of heat which a food is capable of furnishing to the body.
Our bodies are human machines, and, like all other machines, require
fuel for their maintenance. The fuel supplied to an engine is not all
available for pulling the cars; a large portion of the fuel is lost in
smoke, and another portion is wasted as ashes. So it is with the fuel
that runs the body. The food we eat is not all available for
nourishment, much of it being as useless to us as are smoke and ashes
to an engine. The best foods are those which do the most for us with
the least possible waste.

56. Fuel Value. By fuel value is meant the capacity foods have for
yielding heat to the body. The fuel value of the foods we eat daily is
so important a factor in life that physicians, dietitians, nurses,
and those having the care of institutional cooking acquaint themselves
with the relative fuel values of practically all of the important food
substances. The life or death of a patient may be determined by the
patient's diet, and the working and earning capacity of a father
depends largely upon his prosaic three meals. An ounce of fat, whether
it is the fat of meat or the fat of olive oil or the fat of any other
food, produces in the body two and a quarter times as much heat as an
ounce of starch. Of the vegetables, beans provide the greatest
nourishment at the least cost, and to a large extent may be
substituted for meat. It is not uncommon to find an outdoor laborer
consuming one pound of beans per day, and taking meat only on "high
days and holidays."

[Illustration: FIG. 26.--The bomb calorimeter from which the fuel
value of food can be estimated.]

The fuel value of a food is determined by means of the _bomb
calorimeter_ (Fig. 26). The food substance is put into a chamber _A_
and ignited, and the heat of the burning substance raises the
temperature of the water in the surrounding vessel. If 1000 grams of
water are in the vessel, and the temperature of the water is raised 2°
C., the number of calories produced by the substance would be 2000,
and the fuel value would be 2000 calories.[A] From this the fuel value
of one quart or one pound of the substance can be determined, and the
food substance will be said to furnish the body with that number of
heat units, providing all of the pound of food were properly digested.

[Footnote A: As applied to food, the calorie is greater than that used
in the ordinary laboratory work, being the amount of heat necessary to
raise the temperature of 1000 grams of water 1° C., rather than 1 gram
1° C.]

TABLE SHOWING THE NUMBER OF CALORIES FURNISHED BY
ONE POUND OF VARIOUS FOODS
----------------------------------------------------
|FOOD |CALORIES|FOOD |CALORIES|
----------------------------------------------------
|Leg of lean mutton | 790|Carrots | 210|
----------------------------------------------------
|Rib of beef | 1150|Lettuce | 90|
----------------------------------------------------
|Shad | 380|Onion | 225|
----------------------------------------------------
|Chicken | 505|Cucumber | 80|
----------------------------------------------------
|Apples | 290|Almonds | 3030|
----------------------------------------------------
|Bananas | 460|Walnuts | 3306|
----------------------------------------------------
|Prunes | 370|Peanuts | 2560|
----------------------------------------------------
|Watermelons | 140|Oatmeal | 4673|
----------------------------------------------------
|Lima beans | 570|Rolled wheat | 4175|
----------------------------------------------------
|Beets | 215|Macaroni | 1665|
----------------------------------------------------

57. Varied Diet. The human body is a much more varied and complex
machine than any ever devised by man; personal peculiarities, as well
as fuel values, influence very largely the diet of an individual.
Strawberries are excluded from some diets because of a rash which is
produced on the skin, pork is excluded from other diets for a like
reason; cauliflower is absolutely indigestible to some and is readily
digested by others. From practically every diet some foods must be
excluded, no matter what the fuel value of the substance may be.

Then, too, there are more uses for food than the production of heat.
Teeth and bones and nails need a constant supply of mineral matter,
and mineral matter is frequently found in greatest abundance in foods
of low fuel value, such as lettuce, watercress, etc., though
practically all foods yield at least a small mineral constituent. When
fuel values alone are considered, fruits have a low value, but because
of the flavor they impart to other foods, and because of the healthful
influence they exercise in digestion, they cannot be excluded from the
diet.

Care should be constantly exercised to provide substantial foods of
high fuel value. But the nutritive foods should be wisely supplemented
by such foods as fruits, whose real value is one of indirect rather
then direct service.

58. Our Bodies. Somewhat as a house is composed of a group of
bricks, or a sand heap of grains of sand, the human body is composed
of small divisions called cells. Ordinarily we cannot see these cells
because of their minuteness, but if we examine a piece of skin, or a
hair of the head, or a tiny sliver of bone under the microscope, we
see that each of these is composed of a group of different cells. A
merchant, watchful about the fineness of the wool which he is
purchasing, counts with his lens the number of threads to the inch; a
physician, when he wishes, can, with the aid of the microscope,
examine the cells in a muscle, or in a piece of fat, or in a nerve
fiber. Not only is the human body composed of cells, but so also are
the bodies of all animals from the tiny gnat which annoys us, and the
fly which buzzes around us, to the mammoth creatures of the tropics.
These cells do the work of the body, the bone cells build up the
skeleton, the nail cells form the finger and toe nails, the lung cells
take care of breathing, the muscle cells control motion, and the brain
cells are responsible for thought.

59. Why we eat so Much. The cells of the body are constantly, day by
day, minute by minute, breaking down and needing repair, are
constantly requiring replacement by new cells, and, in the case of the
child, are continually increasing in number. The repair of an ordinary
machine, an engine, for example, is made at the expense of money, but
the repair and replacement of our human cell machinery are
accomplished at the expense of food. More than one third of all the
food we eat goes to maintain the body cells, and to keep them in good
order. It is for this reason that we consume a large quantity of food.
If all the food we eat were utilized for energy, the housewife could
cook less, and the housefather could save money on grocer's and
butcher's bills. If you put a ton of coal in an engine, its available
energy is used to run the engine, but if the engine were like the
human body, one third of the ton would be used up by the engine in
keeping walls, shafts, wheels, belts, etc., in order, and only two
thirds would go towards running the engine. When an engine is not
working, fuel is not consumed, but the body requires food for mere
existence, regardless of whether it does active work or not. When we
work, the cells break down more quickly, and the repair is greater
than when we are at rest, and hence there is need of a larger amount
of food; but whether we work or not, food is necessary.

60. The Different Foods. The body is very exacting in its demands,
requiring certain definite foods for the formation and maintenance of
its cells, and other foods, equally definite, but of different
character, for heat; our diet therefore must contain foods of high
fuel value, and likewise foods of cell-forming power.

Although the foods which we eat are of widely different character,
such as fruits, vegetables, cereals, oils, meats, eggs, milk, cheese,
etc., they can be put into three great classes: the carbohydrates, the
fats, and the proteids.

61. The Carbohydrates. Corn, wheat, rye, in fact all cereals and
grains, potatoes, and most vegetables are rich in carbohydrates; as
are also sugar, molasses, honey, and maple sirup. The foods of the
first group are valuable because of the starch they contain; for
example, corn starch, wheat starch, potato starch. The substances of
the second group are valuable because of the sugar they contain; sugar
contains the maximum amount of carbohydrate. In the sirups there is a
considerable quantity of sugar, while in some fruits it is present in
more or less dilute form. Sweet peaches, apples, grapes, contain a
moderate amount of sugar; watermelons, pears, etc., contain less. Most
of our carbohydrates are of plant origin, being found in vegetables,
fruits, cereals, and sirups.

Carbohydrates, whether of the starch group or the sugar group, are
composed chiefly of three elements: carbon, hydrogen, and oxygen; they
are therefore combustible, and are great energy producers. On the
other hand, they are worthless for cell growth and repair, and if we
limited our diet to carbohydrates, we should be like a man who had
fuel but no engine capable of using it.

62. The Fats. The best-known fats are butter, lard, olive oil, and
the fats of meats, cheese, and chocolate. When we test fats for fuel
values by means of a calorimeter (Fig. 26), we find that they yield
twice as much heat as the carbohydrates, but that they burn out more
quickly. Dwellers in cold climates must constantly eat large
quantities of fatty foods if they are to keep their bodies warm and
survive the extreme cold. Cod liver oil is an excellent food medicine,
and if taken in winter serves to warm the body and to protect it
against the rigors of cold weather. The average person avoids fatty
foods in summer, knowing from experience that rich foods make him warm
and uncomfortable. The harder we work and the colder the weather, the
more food of that kind do we require; it is said that a lumberman
doing heavy out-of-door work in cold climates needs three times as
much food as a city clerk. Most of our fats, like lard and butter, are
of animal origin; some of them, however, like olive oil, peanut
butter, and coconut oil, are of plant origin.

[Illustration: FIG. 27.--_a_ is the amount of fat necessary to make
one calorie; _b_ is the amount of sugar or proteid necessary to make
one calorie.]

63. The Proteids. The proteids are the building foods, furnishing
muscle, bone, skin cells, etc., and supplying blood and other bodily
fluids. The best-known proteids are white of egg, curd of milk, and
lean of fish and meat; peas and beans have an abundant supply of this
substance, and nuts are rich in it. Most of our proteids are of animal
origin, but some protein material is also found in the vegetable
world. This class of foods contains carbon, oxygen, and hydrogen, and
in addition, two substances not found in carbohydrates or
fats--namely, sulphur and nitrogen. Proteids always contain nitrogen,
and hence they are frequently spoken of as nitrogenous foods. Since
the proteids contain all the elements found in the two other classes
of foods, they are able to contribute, if necessary, to the store of
bodily energy; but their main function is upbuilding, and the diet
should be chosen so that the proteids do not have a double task.

For an average man four ounces of dry proteid matter daily will
suffice to keep the body cells in normal condition.

It has been estimated that 300,000,000 blood cells alone need daily
repair or renewal. When we consider that the blood is but one part of
the body, and that all organs and fluids have corresponding
requirements, we realize how vast is the work to be done by the food
which we eat.

64. Mistakes in Buying. The body demands a daily ration of the three
classes of food stuffs, but it is for us to determine from what
meats, vegetables, fruits, cereals, etc., this supply shall be
obtained (Figs. 28 and 29).

[Illustration: FIG. 28.--Table of food values.]

[Illustration: FIG. 29.--Diagram showing the difference in the cost of
three foods which give about the same amount of nutrition each.]

Generally speaking, meats are the most expensive foods we can
purchase, and hence should be bought seldom and in small quantities.
Their place can be taken by beans, peas, potatoes, etc., and at less
than a quarter of the cost. The average American family eats meat
three times a day, while the average family of the more conservative
and older countries rarely eats meat more than once a day. The
following tables indicate the financial loss arising from an unwise
selection of foods:--

FOOD CONSUMED--ONE WEEK
|===========================|=======================================|
| FAMILY No. 1 | || FAMILY No. 2 |
|---------------------------|---------------------------------------|
|20 loaves of bread | $1.00 ||15 lb. flour, bread |
|10 to 12 lb. loin steak | || home made (skim milk used) | $.45
| or meat of similar cost | 2.00 ||Yeast, shortening, and |
|20 to 25 lb. rib roast | || skim milk | .10
| or similar meat | 4.40 ||10 lb. steak (round, Hamburger|
|4 lb. high-priced cereal | || and some loin) | 1.50
| breakfast food, 20¢ | .80 ||10 lb. other meats, boiling |
|Cake and pastry purchased | 3.00 || pieces, rump roast, etc. | 1.00
|8 lb. butter, 30¢ | 2.40 || 5 lb. cheese, 16¢ | .80
|Tea, coffee, spices, etc. | .75 || 5 lb. oatmeal (bulk) | .15
|Mushrooms | .75 || 5 lb. beans | .25
|Celery | 1.00 ||Home-made cake and pastry | 1.00
|Oranges | 2.00 || 6 lb. butter, 30¢ | 1.80
|Potatoes | .25 || 3 lb. home-made shortening | .25
|Miscellaneous canned goods | 2.00 ||Tea, coffee, and spices | .40
|Milk | .50 ||Apples | .50
|Miscellaneous foods | 2.00 ||Prunes | .25
|3 doz. eggs | .60 ||Potatoes | .25
| |-------||Milk | 1.00
| |$23.45 ||Miscellaneous foods | 1.00
| | || 3 doz. eggs | .60
| | || -|-----
| | || $|11.30
| -----------------------|-----------------------------------------|---
| -----------------------|-----------------------------------------|---

"The tables show that one family spends over twice as much in the
purchase of foods as the other family, and yet the one whose food
costs the less actually secures the larger amount of nutritive
material and is better fed than the family where more money is
expended."--From _Human Foods_, Snyder.

The Source of the Different Foods. All of our food comes from either
the plant world or the animal world. Broadly speaking, plants furnish
the carbohydrates, that is, starch and sugar; animals furnish the fats
and proteids. But although vegetable foods yield carbohydrates mainly,
some of them, like beans and peas, contain large quantities of protein
and can be substituted for meat without disadvantage to the body.
Other plant products, such as nuts, have fat as their most abundant
food constituent. The peanut, for example, contains 43% of fat, 30% of
proteids, and only 17% of carbohydrates; the Brazil nut has 65% of
fat, 17% of proteids, and only 9% of carbohydrates. Nuts make a good
meat substitute, and since they contain a fair amount of carbohydrates
besides the fats and proteins, they supply all of the essential food
constituents and form a well-balanced food.




CHAPTER VI

WATER


65. Destructive Action of Water. The action of water in stream and
sea, in springs and wells, is evident to all; but the activity of
ground water--that is, rain water which sinks into the soil and
remains there--is little known in general. The real activity of ground
water is due to its great solvent power; every time we put sugar into
tea or soap into water we are using water as a solvent. When rain
falls, it dissolves substances floating in the atmosphere, and when it
sinks into the ground and becomes ground water, it dissolves material
out of the rock which it encounters (Fig. 30). We know that water
contains some mineral matter, because kettles in which water is boiled
acquire in a short time a crust or coating on the inside. This crust
is due to the accumulation in the kettle of mineral matter which was
in solution in the water, but which was left behind when the water
evaporated. (See Section 25.)

[Illustration: FIG. 30.--Showing how caves and holes are formed by the
solvent action of water.]

The amount of dissolved mineral matter present in some wells and
springs is surprisingly great; the famous springs of Bath, England,
contain so much mineral matter in solution, that a column 9 feet in
diameter and 140 feet high could be built out of the mineral matter
contained in the water consumed yearly by the townspeople.

[Illustration: FIG. 31.--The work of water as a solvent.]

Rocks and minerals are not all equally soluble in water; some are so
little soluble that it is years before any change becomes apparent,
and the substances are said to be insoluble, yet in reality they are
slowly dissolving. Other rocks, like limestone, are so readily soluble
in water that from the small pores and cavities eaten out by the
water, there may develop in long centuries, caves and caverns (Fig.
30). Most rock, like granite, contains several substances, some of
which are readily soluble and others of which are not readily soluble;
in such rocks a peculiar appearance is presented, due to the rapid
disappearance of the soluble substance, and the persistence of the
more resistant substance (Fig. 31).

We see that the solvent power of water is constantly causing changes,
dissolving some mineral substances, and leaving others practically
untouched; eating out crevices of various shapes and sizes, and by
gradual solution through unnumbered years enlarging these crevices
into wonderful caves, such as the Mammoth Cave of Kentucky.

66. Constructive Action of Water. Water does not always act as a
destructive agent; what it breaks down in one place it builds up in
another. It does this by means of precipitation. Water dissolves salt,
and also dissolves lead nitrate, but if a salt solution is mixed with
a lead nitrate solution, a solid white substance is formed in the
water (Fig. 32). This formation of a solid substance from the mingling
of two liquids is called precipitation; such a process occurs daily in
the rocks beneath the surface of the earth. (See Laboratory Manual.)

[Illustration: FIG. 32.--From the mingling of two liquids a solid is
sometimes formed.]

Suppose water from different sources enters a crack in a rock,
bringing different substances in solution; then the mingling of the
waters may cause precipitation, and the solid thus formed will be
deposited in the crack and fill it up. Hence, while ground water tends
to make rock porous and weak by dissolving out of it large quantities
of mineral matter, it also tends under other conditions to make it
more compact because it deposits in cracks, crevices, and pores the
mineral matter precipitated from solution.

These two forces are constantly at work; in some places the
destructive action is more prominent, in other places the constructive
action; but always the result is to change the character of the
original substance. When the mineral matter precipitated from the
solutions is deposited in cracks, _veins_ are formed (Fig. 33), which
may consist of the ore of different metals, such as gold, silver,
copper, lead, etc. Man is almost entirely dependent upon these veins
for the supply of metal needed in the various industries, because in
the original condition of the rocks, the metallic substances are so
scattered that they cannot be profitably extracted.

[Illustration: FIG. 33.--Mineral matter precipitated from solution is
deposited in crevices and forms veins.]

Naturally, the veins themselves are not composed of one substance
alone, because several different precipitates may be formed. But there
is a decided grouping of valuable metals, and these can then be
readily separated by means of electricity.

67. Streams. Streams usually carry mud and sand along with them;
this is particularly well seen after a storm when rivers and brooks
are muddy. The puddles which collect at the foot of a hill after a
storm are muddy because of the particles of soil gathered by the water
as it runs down the hill. The particles are not dissolved in the
water, but are held there in suspension, as we call it technically.
The river made muddy after a storm by suspended particles usually
becomes clear and transparent after it has traveled onward for miles,
because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne
along and deposited at various places (Fig. 34). The amount of
deposition by large rivers is so great that in some places channels
fill up and must be dredged annually, and vessels are sometimes caught
in the deposit and have to be towed away.

Running water in the form of streams and rivers, by carrying sand
particles, stones, and rocks from high slopes and depositing them at
lower levels, wears away land at one place and builds it up at
another, and never ceases in its work of changing the nature of the
earth's surface (Fig. 35).

[Illustration: FIG. 34.--Deposit left by running water.]

[Illustration: FIG. 35.--Water by its action constantly changes the
character of the land.]

68. Relation of Water to Human Life. Water is one of the most
essential of food materials, and whether we drink much or little
water, we nevertheless get a great deal of it. The larger part of many
of our foods is composed of water; more than half of the weight of the
meat we eat is made up of water; and vegetables are often more than
nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes
have over 90 per cent. of water, and most fruits are more than three
fourths water; even bread, which contains as little water as any of
our common foods, is about one third water (Fig. 36).

[Illustration: FIG. 36.--Diagram of the composition of a loaf of bread
and of a potato: 1. ash; 2, food; 3, water.]

Without water, solid food material, although present in the body,
would not be in a condition suitable for bodily use. An abundant
supply of water enables the food to be dissolved or suspended in it,
and in solution the food material is easily distributed to all parts
of the body.

Further, water assists in the removal of the daily bodily wastes, and
thus rids the system of foul and poisonous substances.

The human body itself consists largely of water; indeed, about two
thirds of our own weight is water. The constant replenishing of this
large quantity is necessary to life, and a considerable amount of the
necessary supply is furnished by foods, particularly the fruits and
vegetables.

But while the supply furnished by the daily food is considerable, it
is by no means sufficient, and should be supplemented by good drinking
water.

69. Water and its Dangers. Our drinking water comes from far and
near, and as it moves from place to place, it carries with it in
solution or suspension anything which it can find, whether it be
animal, vegetable, or mineral matter. The power of water to gather up
matter is so great that the average drinking water contains 20 to 90
grains of solid matter per gallon; that is, if a gallon of ordinary
drinking water is left to evaporate, a residue of 20 to 90 grains will
be left. (See Laboratory Manual.) As water runs down a hill slope
(Fig. 37), it carries with it the filth gathered from acres of land;
carries with it the refuse of stable, barn, and kitchen; and too often
this impure surface water joins the streams which supply our cities.
Lakes and rivers which furnish drinking water should be carefully
protected from surface draining; that is, from water which has flowed
over the land and has thus accumulated the waste of pasture and
stable and, it may be, of dumping ground.

[Illustration: FIG. 37.--As water flows over the land, it gathers
filth and disease germs.]

It is not necessary that water should be absolutely free from all
foreign substances in order to be safe for daily use in drinking; a
limited amount of mineral matter is not injurious and may sometimes be
really beneficial. It is the presence of animal and vegetable matter
that causes real danger, and it is known that typhoid fever is due
largely to such impurities present in the drinking water.

70. Methods of Purification. Water is improved by any of the
following methods:--

(_a_) _Boiling_. The heat of boiling destroys animal and vegetable
germs. Hence water that has been boiled a few minutes is safe to use.
This is the most practical method of purification in the home, and is
very efficient. The boiled water should be kept in clean, corked
bottles; otherwise foreign substances from the atmosphere reënter the
water, and the advantage gained from boiling is lost.

(_b_) _Distillation_. By this method pure water is obtained, but this
method of purification cannot be used conveniently in the home
(Section 25).

(_c_) _Filtration_. In filtration, the water is forced through
porcelain or other porous substances which allow the passage of water,
but which hold back the minute foreign particles suspended in the
water. (See Laboratory Manual.) The filters used in ordinary dwellings
are of stone, asbestos, or charcoal. They are often valueless, because
they soon become choked and cannot be properly cleaned.

The filtration plants owned and operated by large cities are usually
safe; there is careful supervision of the filters, and frequent and
effective cleanings are made. In many cities the filtration system is
so good that private care of the water supply is unnecessary.

71. The Source of Water. In the beginning, the earth was stored with
water just as it was with metal, rock, etc. Some of the water
gradually took the form of rivers, lakes, streams, and wells, as now,
and it is this original supply of water which furnishes us all that we
have to-day. We quarry to obtain stone and marble for building, and we
fashion the earth's treasures into forms of our own, but we cannot
create these things. We bore into the ground and drill wells in order
to obtain water from hidden sources; we utilize rapidly flowing
streams to drive the wheels of commerce, but the total amount of water
remains practically unchanged.

The water which flows on the earth is constantly changing its form;
the heat of the sun causes it to evaporate, or to become vapor, and to
mingle with the atmosphere. In time, the vapor cools, condenses, and
falls as snow or rain; the water which is thus returned to the earth
feeds our rivers, lakes, springs, and wells, and these in turn supply
water to man. When water falls upon a field, it soaks into the ground,
or collects in puddles which slowly evaporate, or it runs off and
drains into small streams or into rivers. That which soaks into the
ground is the most valuable because it remains on the earth longest
and is the purest.

[Illustration: FIG. 38.--How springs are formed. _A_, porous layer;
_B_, non-porous layer; _C_, spring.]

Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock
through which it cannot pass, and which effectually hinders its
downward passage. In such regions, there is an accumulation of water,
and a well dug there would have an abundant supply of water. The
non-porous layer is rarely level, and hence the water whose vertical
path is obstructed does not "back up" on the soil, but flows down hill
parallel with the obstructing non-porous layer, and in some distant
region makes an outlet for itself, forming a spring (Fig. 38). The
streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams
and rivers evaporation occurs, the water once more becomes vapor and
passes into the atmosphere, where it is condensed and again falls to
the earth.

Water which has filtered through many feet of earth is far purer and
safer than that which fell directly into the rivers, or which ran off
from the land and joined the surface streams without passing through
the soil.

72. The Composition of Water. Water was long thought to be a simple
substance, but toward the end of the eighteenth century it was found
to consist of two quite different substances, oxygen (O) and hydrogen
(H.)

[Illustration: FIG. 39.--The decomposition of water.]

If we send an electric current through water (acidulated to make it a
good conductor), as shown in Figure 39, we see bubbles of gas rising
from the end of the wire by which the current enters the water, and
other bubbles of gas rising from the end of the wire by which the
current leaves the water. These gases have evidently come from the
water and are the substances of which it is composed, because the
water begins to disappear as the gases are formed. If we place over
each end of the wire an inverted jar filled with water, the gases are
easily collected. The first thing we notice is that there is always
twice as much of one gas as of the other; that is, water is composed
of two substances, one of which is always present in twice as large
quantities as the other.

73. The Composition of Water. On testing the gases into which water
is broken up by an electric current, we find them to be quite
different. One proves to be oxygen, a substance with which we are
already familiar. The other gas, hydrogen, is new to us and is
interesting as being the lightest known substance, being even "lighter
than a feather."

An important fact about hydrogen is that in burning it gives as much
heat as five times its weight of coal. Its flame is blue and almost
invisible by daylight, but intensely hot. If fine platinum wire is
placed in an ordinary gas flame, it does not melt, but if placed in a
flame of burning hydrogen, it melts very quickly.

74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour
sulphuric acid, or hydrochloric acid, on zinc shavings and to collect
in a bottle the gas which is given off. This gas proves to be
colorless, tasteless, and odorless. (See Laboratory Manual.)




CHAPTER VII

AIR


75. The Instability of the Air. We are usually not conscious of the
air around us, but sometimes we realize that the air is heavy, while
at other times we feel the bracing effect of the atmosphere. We live
in an ocean of air as truly as fish inhabit an ocean of water. If you
have ever been at the seashore you know that the ocean is never still
for a second; sometimes the waves surge back and forth in angry fury,
at other times the waves glide gently in to the shore and the surface
is as smooth as glass; but we know that there is perpetual motion of
the water even when the ocean is in its gentlest moods. Generally our
atmosphere is quiet, and we are utterly unconscious of it; at other
times we are painfully aware of it, because of its furious winds. Then
again we are oppressed by it because of the vast quantity of vapor
which it holds in the form of fog, or mist. The atmosphere around us
is as restless and varying as is the water of the sea. The air at the
top of a high tower is very different from the air at the base of the
tower. Not only does the atmosphere vary greatly at different
altitudes, but it varies at the same place from time to time, at one
period being heavy and raw, at another being fresh and invigorating.

Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.

76. The Character of the Air. The atmosphere which envelops us at
all times extends more than fifty miles above us, its height being far
greater than the greatest depths of the sea. This atmosphere varies
from place to place; at the sea level it is heavy, on the mountain top
less heavy, and far above the earth it is so light that it does not
contain enough oxygen to permit man to live. Figure 40 illustrates by
a pile of pillows how the pressure of the air varies from level to
level.

[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]

Sea level is a low portion of the earth's surface, hence at sea level
there is a high column of air, and a heavy air pressure. As one passes
from sea level to mountain top a gradual but steady decrease in the
height of the air column occurs, and hence a gradual but definite
lessening of the air pressure.

[Illustration: FIG. 41.--The water in the tube is at the same level as
that in the glass.]

77. Air Pressure. If an empty tube (Fig. 41) is placed upright in
water, the water will not rise in the tube, but if the tube is put in
water and the air is then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what happens when we
take lemonade through a straw. When the air is withdrawn from the
straw by the mouth, the pressure within the straw is reduced, and the
liquid is forced up the straw by the air pressure on the surface of
the liquid in the glass. Even the ancient Greeks and Romans knew that
water would rise in a tube when the pressure within the tube was
reduced, and hence they tried to obtain water from wells in this
fashion, but the water could never be raised higher than 34 feet. Let
us see why water could rise 34 feet and no more. If an empty pipe is
placed in a cistern of water, the water in the pipe does not rise
above the level of the water in the cistern. If, however, the pressure
in the tube is removed, the water in the tube will rise to a height of
34 feet approximately. If now the air pressure in the tube is
restored, the water in the tube sinks again to the level of that in
the cistern. The air pressing on the liquid in the cistern tends to
push some liquid up the tube, but the air pressing on the water in the
tube pushes downwards, and tends to keep the liquid from rising, and
these two pressures balance each other. When, however, the pressure
within the tube is reduced, the liquid rises because of the unbalanced
pressure which acts on the water in the cistern.

[Illustration: FIG. 42.--Water rises in the tube when the air is
withdrawn.]

[Illustration: FIG. 43.--The air supports a column of mercury 30
inches high.]

The column of water which can be raised this way is approximately 34
feet, sometimes a trifle more, sometimes a trifle less. If water were
twice as heavy, just half as high a column could be supported by the
atmosphere. Mercury is about thirteen times as heavy as water and,
therefore, the column of mercury supported by the atmosphere is about
one thirteenth as high as the column of water supported by the
atmosphere. This can easily be demonstrated. Fill a glass tube about a
yard long with mercury, close the open end with a finger, and quickly
insert the end of the inverted tube in a dish of mercury (Fig. 43).
When the finger is removed, the mercury falls somewhat, leaving an
empty space in the top of the tube. If we measure the column in the
tube, we find its height is about one thirteenth of 34 feet or 30
inches, exactly what we should expect. Since there is no air pressure
within the tube, the atmospheric pressure on the mercury in the dish
is balanced solely by the mercury within the tube, that is, by a
column of mercury 30 inches high. The shortness of the mercury column
as compared with that of water makes the mercury more convenient for
both experimental and practical purposes. (See Laboratory Manual.)

78. The Barometer. Since the pressure of the air changes from time
to time, the height of the mercury will change from day to day, and
hour to hour. When the air pressure is heavy, the mercury will tend to
be high; when the air pressure is low, the mercury will show a shorter
column; and by reading the level of the mercury one can learn the
pressure of the atmosphere. If a glass tube and dish of mercury are
attached to a board and the dish of mercury is inclosed in a case for
protection from moisture and dirt, and further if a scale of inches or
centimeters is made on the upper portion of the board, we have a
mercurial barometer (Fig. 44).

[Illustration: FIG. 44.--A simple barometer.]

If the barometer is taken to the mountain top, the column of mercury
falls gradually during the ascent, showing that as one ascends, the
pressure decreases in agreement with the statement in Section 76.
Observations similar to these were made by Torricelli as early as the
sixteenth century. Taking a barometric reading consists in measuring
the height of the mercury column.

79. A Portable Barometer. The mercury barometer is large and
inconvenient to carry from place to place, and a more portable form
has been devised, known as the aneroid barometer (Fig. 45). This form
of barometer is extremely sensitive; indeed, it is so delicate that
it shows the slight difference between the pressure at the table top
and the pressure at the floor level, whereas the mercury barometer
would indicate only a much greater variation in atmospheric pressure.
The aneroid barometers are frequently made no larger than a watch and
can be carried conveniently in the pocket, but they get out of order
easily and must be frequently readjusted. The aneroid barometer is an
air-tight box whose top is made of a thin metallic disk which bends
inward or outward according to the pressure of the atmosphere. If the
atmospheric pressure increases, the thin disk is pushed slightly
inward; if, on the other hand, the atmospheric pressure decreases, the
pressure on the metallic disk decreases and the disk is not pressed so
far inward. The motion of the disk is small, and it would be
impossible to calculate changes in atmospheric pressure from the
motion of the disk, without some mechanical device to make the slight
changes in motion perceptible.

[Illustration: FIG. 45.--Aneroid barometer.]

In order to magnify the slight changes in the position of the disk,
the thin face is connected with a system of levers, or wheels, which
multiplies the changes in motion and communicates them to a pointer
which moves around a graduated circular face. In Figure 45 the real
barometer is scarcely visible, being securely inclosed in a metal case
for protection; the principle, however, can be understood by reference
to Figure 46.

[Illustration: FIG. 46.--Principle of the aneroid barometer.]

80. The Weight of the Air. We have seen that the pressure of the
atmosphere at any point is due to the weight of the air column which
stretches from that point far up into the sky above. This weight
varies slightly from time to time and from place to place, but it is
equal to about 15 pounds to the square inch as shown by actual
measurement. It comes to us as a surprise sometimes that air actually
has weight; for example, a mass of 12 cubic feet of air at average
pressure weighs 1 pound, and the air in a large assembly hall weighs
more than 1 ton.

We are practically never conscious of this really enormous pressure of
the atmosphere, which is exerted over every inch of our bodies,
because the pressure is exerted equally over the outside and the
inside of our bodies; the cells and tissues of our bodies containing
gases under atmospheric pressure. If, however, the finger is placed
over the open end of a tube and the air is sucked out of the tube by
the mouth, the flesh of the finger bulges into the tube because the
pressure within the finger is no longer equalized by the usual
atmospheric pressure (Fig. 47).

[Illustration: FIG. 47.--The flesh bulges out.]

Aëronauts have never ascended much higher than 7 miles; at that height
the barometer stands at 7 inches instead of at 30 inches, and the
internal pressure in cells and tissues is not balanced by an equal
external pressure. The unequalized internal pressure forces the blood
to the surface of the body and causes rupture of blood vessels and
other physical difficulties.

81. Use of the Barometer. Changes in air pressure are very closely
connected with changes in the weather. The barometer does not directly
foretell the weather, but a low or falling pressure, accompanied by a
simultaneous fall of the mercury, usually precedes foul weather, while
a rising pressure, accompanied by a simultaneous rise in the mercury,
usually precedes fair weather. The barometer is not an infallible
prophet, but it is of great assistance in predicting the general trend
of the weather. There are certain changes in the barometer which
follow no known laws, and which allow of no safe predictions, but on
the other hand, general future conditions for a few days ahead can be
fairly accurately determined. Figure 48 shows a barograph or
self-registering barometer which automatically registers air pressure.

[Illustration: FIG. 48.--Barograph.]

Seaport towns in particular, but all cities, large or small, and
villages too, are on request notified by the United States Weather
Bureau ten hours or more in advance, of probable weather conditions,
and in this way precautions are taken which annually save millions of
dollars and hundreds of lives.

I recollect a summer spent on a New Hampshire farm, and know that an
old farmer started his farm hands haying by moonlight at two o'clock
in the morning, because the Special Farmer's Weather Forecast of the
preceding evening had predicted rain for the following day. His
reliance on the weather report was not misplaced, since the storm came
with full force at noon. Sailing vessels, yachts, and fishing dories
remain within reach of port if the barometer foretells storms.

[Illustration: FIG. 49.--Isotherms.]

82. Isobaric and Isothermal Lines. If a line were drawn through all
points on the surface of the earth having an equal barometric pressure
at the same time, such a line would be called an isobar. For example,
if the height of barometers in different localities is observed at
exactly the same time, and if all the cities and towns which have the
same pressure are connected by a line, the curved lines will be called
isobars. By the aid of these lines the barometric conditions over a
large area can be studied. The Weather Bureau at Washington relies
greatly on these isobars for statements concerning local and distant
weather forecasts, any shift in isobaric lines showing change in
atmospheric pressure.

If a line is drawn through all points on the surface of the earth
having the same _temperature_ at the same instant, such a line is
called an isotherm (Fig. 49).

83. Weather Maps. Scattered over the United States are about 125
Government Weather Stations, at each of which three times a day, at
the same instant, accurate observations of the weather are made. These
observations, which consist of the reading of barometer and
thermometer, the determination of the velocity and direction of the
wind, the determination of the humidity and of the amount of rain or
snow, are telegraphed to the chief weather official at Washington.
From the reports of wind storms, excessive rainfall, hot waves,
clearing weather, etc., and their rate of travel, the chief officials
predict where the storms, etc., will be at a definite future time. In
the United States, the _general_ movement of weather conditions, as
indicated by the barometer, is from west to east, and if a certain
weather condition prevails in the west, it is probable that it will
advance eastward, although with decided modifications. So many
influences modify atmospheric conditions that unfailing predictions
are impossible, but the Weather Bureau predictions prove true in about
eight cases out of ten.

The reports made out at Washington are telegraphed on request to
cities in this country, and are frequently published in the daily
papers, along with the forecast of the local office. A careful study
of these reports enables one to forecast to some extent the probable
weather conditions of the day.

The first impression of a weather map (Fig. 50) with its various lines
and signals is apt to be one of confusion, and the temptation comes to
abandon the task of finding an underlying plan of the weather. If one
will bear in mind a few simple rules, the complexity of the weather
map will disappear and a glance at the map will give one information
concerning general weather conditions just as a glance at the
thermometer in the morning will give some indication of the probable
temperature of the day. (See Laboratory Manual.)

[Illustration: FIG. 50. weather Map]

On the weather map solid lines represent isobars and dotted lines
represent isotherms. The direction of the wind at any point is
indicated by an arrow which flies with the wind; and the state of the
weather--clear, partly cloudy, cloudy, rain, snow, etc.--is indicated
by symbols.

84. Components of the Air. The best known constituent of the air is
oxygen, already familiar to us as the feeder of the fire without and
within the body. Almost one fifth of the air which envelops us is made
up of the life-giving oxygen. This supply of oxygen in the air is
constantly being used up by breathing animals and glowing fires, and
unless there were some constant source of additional supply, the
quantity of oxygen in the air would soon become insufficient to
support animal life. The unfailing constant source of atmospheric
oxygen is plant life (Section 48). The leaves of plants absorb carbon
dioxide from the air, and break it up into oxygen and carbon. The
plant makes use of the carbon but it rejects the oxygen, which passes
back into the atmosphere through the pores of the leaves.

Although oxygen constitutes only one fifth of the atmosphere, it is
one of the most abundant and widely scattered of all substances.
Almost the whole earth, whether it be rich loam, barren clay, or
granite boulder, contains oxygen in some form or other; that is, in
combination with other substances. But nowhere, except in the air
around us, do we find oxygen free and uncombined with other
substances.

A less familiar but more abundant constituent of the atmosphere is the
nitrogen. Almost four fifths of the air around us is made up of
nitrogen. If the atmosphere were composed of oxygen alone, the merest
flicker of a match would set the whole world ablaze. The fact that the
oxygen of the air is diluted as it were with so large a proportion of
nitrogen, prevents fires from sweeping over the world and destroying
everything in their path. Nitrogen does not support combustion, and a
burning match placed in a corked bottle goes out as soon as it has
used up the oxygen in the bottle. The nitrogen in the bottle, not only
does not assist the burning of the match, but it acts as a damper to
the burning.

Free nitrogen, like oxygen, is a colorless, odorless gas. It is not
poisonous; but one would die if surrounded by nitrogen alone, just as
one would die if surrounded by water. The vast supply of nitrogen in
the atmosphere would be useless if the smaller amount of oxygen were
not present to keep the body alive. Nitrogen is so important a factor
in daily life that an entire chapter will be devoted to it later.

Another constituent of the air with which we are familiar is carbon
dioxide. In pure air, carbon dioxide is present in very small
proportion, being continually taken from the air by plants in the
manufacture of their food.

Various other substances are present in the air in very minute
proportions, but of all the substances in the air, oxygen, nitrogen,
and carbon dioxide are the most important.




CHAPTER VIII

GENERAL PROPERTIES OF GASES


85. Bicycle Tires. We know very well that we cannot put more than a
certain amount of water in a tube, but we know equally well that the
amount of air which can be pumped into a bicycle or automobile tire
depends largely upon our muscular energy. A gallon of water remains a
gallon of water and requires a perfectly definite amount of space, but
air can be compressed and compressed, and made to occupy less and less
space. While it is true that air is easily compressed, it is also true
that air is elastic and capable of very rapid and easy expansion. If a
puncture occurs in a tire, the compressed air escapes very quickly;
that is, the compressed air within the tube has taken the first
opportunity offered for expansion.

[Illustration: FIG. 51.--By squeezing the bulb, air is forced out of
the nozzle.]

The fact that air is elastic has added materially to the comfort of
the world. Transportation by bicycles and automobiles has been greatly
facilitated by the use of air tires. In many hospitals, air mattresses
are used in place of hair, feather, or cotton mattresses, and in this
way the bed is kept fresher and cleaner, and can be moved with less
danger of discomfort to the patient. Every time we squeeze the bulb of
an atomizer, we force compressed or condensed air through the
atomizer, and the condensed air pushes the liquid out of the nozzle
(Fig. 51). Thus we see that in the necessities and conveniences of
life compressed air plays an important part.

86. The Danger of Compression. Air under ordinary atmospheric
conditions exerts a pressure of 15 pounds to the square inch. If, now,
large quantities of air are compressed into a small space, the
pressure exerted becomes correspondingly greater. If too much air is
blown into a toy balloon, the balloon bursts because it cannot support
the great pressure exerted by the compressed air within. What is true
of air is true of all gases. Dangerous boiler explosions have occurred
because the boiler walls were not strong enough to withstand the
pressure of the steam (which is water in the form of gas). The
pressure within the boilers of engines is frequently several hundred
pounds to the square inch, and such a pressure needs a strong boiler.

87. How Pressure is Measured in Buildings. In the preceding Section
we saw that undue pressure of a gas may cause explosion. It is
important, therefore, that authorities keep strict watch on gases
confined within pipes and reservoirs, never allowing the pressure to
exceed that which the walls of the reservoir will safely bear.

[Illustration: FIG. 52.--A pressure gauge.]

Pressure in a gas pipe may be measured by a simple instrument called
the pressure gauge: The gauge consists of a bent glass tube containing
mercury, and so made that one end can be fitted to a gas jet (Fig.
52). When the gas cock is closed, the mercury stands at the same level
in both arms, but when the cock is opened, the gas whose pressure is
being measured forces the mercury up the opposite arm. If the pressure
of the gas is small, the mercury changes its level but very little. It
is clear that the height of a column of mercury is a measure of the
gas pressure. Now it is known that one cubic inch of mercury weighs
about half a pound. Hence a column of mercury one inch high indicates
a pressure of about one half pound to the square inch; a column two
inches high indicates a pressure of about one pound to the square
inch, and so on.

This is a very convenient way to measure the pressure of the
illuminating gas in our homes and offices. The gauge is attached to
the gas burner and the pressure is read by means of a scale attached
to the gauge. (See Laboratory Manual.)

In order to have satisfactory illumination, the pressure must be
strong enough to give a steady, broad flame. If the flame from any gas
jet is flickering and weak, it is usually an indication of
insufficient pressure and the gas company should investigate
conditions and see to it that the consumer receives his proper value.

87. The Gas Meter. Most householders are deeply interested in the
actual amount of gas which they consume (gas is charged for according
to the number of cubic feet used), and therefore they should be able
to read the gas meter which indicates their consumption of gas. Such
gas meters are furnished by the companies, and can be read easily.

[Illustration: FIG. 53.--The gas meter indicates the number of cubic
feet of gas consumed.]

The instrument itself is somewhat complex. It will suffice to say that
within the meter box are thin disks which are moved by the stream of
gas that passes them. This movement of the disks is recorded by
clockwork devices on a dial face. In this way, the number of cubic
feet of gas which pass through the meter is automatically registered.

89. The Relation between Pressure and Volume. It was long known that
as the pressure of a gas increases, that is, as it becomes compressed,
its volume decreases, but Robert Boyle was the first to determine the
exact relation between the volume and the pressure of a gas. He did
this in a very simple manner.

Pour mercury into a U-shaped tube until the level of the mercury in
the closed end of the tube is the same as the level in the open end.
The air in the long arm is pressing upon the mercury in that arm, and
is tending to force it up the short arm. The air in the short closed
arm is pressing down upon the mercury in that arm and tending to send
it up the long arm. Since the mercury is at the same level in the two
arms, the pressure in the long arm must be equal to the pressure in
the short arm. But the long arm is open, and the pressure in that arm
is the pressure of the atmosphere. Therefore the pressure in the short
arm must be one atmosphere. Measure the distance _bc_ between the top
of the mercury and the closed end of the tube.

[Illustration: FIGS. 54, 55.--As the pressure on the gas increases,
its volume decreases.]

Pour more mercury into the open end of the tube, and as the mercury
rises higher and higher in the long arm, note carefully the decrease
in the volume of the air in the short arm. Pour mercury into the tube
until the difference in level _bd_ is just equal to the barometric
height, approximately 32 inches. The pressure of the air in the closed
end now supports the pressure of one atmosphere, and in addition, a
column of mercury equal to another atmosphere. If now the air column
in the closed end is measured, its volume will be only one half of its
former volume. By doubling the pressure we have reduced the volume one
half. Similarly, if the pressure is increased threefold, the volume
will be reduced to one third of the original volume.

90. Heat due to Compression. We saw in Section 89 that whenever the
pressure exerted upon a gas is increased, the volume of the gas is
decreased; and that whenever the pressure upon a gas is decreased, the
volume of the gas is increased. If the pressure is changed very
slowly, the change in the temperature of the gas is imperceptible; if,
however, the pressure is removed suddenly, the temperature falls
rapidly, or if the pressure is applied suddenly, the temperature rises
rapidly. When bicycle tires are being inflated, the pump becomes hot
because of the compression of the air.

The amount of heat resulting from compression is surprisingly large;
for example, if a mass of gas at 0° C. is suddenly compressed to one
half its original volume, its temperature rises 87° C.

91. Cooling by Expansion. If a gas expands suddenly, its temperature
falls; for example, if a mass of gas at 87° C. is allowed to expand
rapidly to twice its original volume, its temperature falls to 0° C.
If the compressed air of a bicycle tire is allowed to expand and a
sensitive thermometer is held in the path of the escaping air, the
thermometer will show a decided drop in temperature.

The low temperature obtained by the expansion of air or other gases is
utilized commercially on a large scale. By means of powerful pistons
air is compressed to one third or one fourth its original volume, is
passed through a coil of pipe surrounded with cold water, and is then
allowed to escape into large refrigerating vaults, which thereby have
their temperatures noticeably lowered, and can be used for the
permanent storage of meats, fruits, and other perishable material. In
summer, when the atmospheric temperature is high, the storage and
preservation of foods is of vital importance to factories and cold
storage houses, and but for the low temperature obtainable by the
expansion of compressed gases, much of our food supply would be lost
to use.

92. Unexpected Transformations. If the pressure on a gas is greatly
increased, a sudden transformation sometimes occurs and the gas
becomes a liquid. Then, if the pressure is reduced, a second
transformation occurs, and the liquid evaporates or returns to its
original form as a gas.

In Section 23 we saw that a fall of temperature caused water vapor to
condense or liquefy. If temperature alone were considered, most gases
could not be liquefied, because the temperature at which the average
gas liquefies is so low as to be out of the range of possibility; it
has been calculated, for example, that a temperature of 252° C. below
zero would have to be obtained in order to liquefy hydrogen.

Some gases can be easily transformed into liquids by pressure alone,
some gases can be easily transformed into liquids by cooling alone; on
the other hand, many gases are so difficult to liquefy that both
pressure and low temperature are needed to produce the desired result.
If a gas is cooled and compressed at the same time, liquefaction
occurs much more surely and easily than though either factor alone
were depended upon. The air which surrounds us, and of whose existence
we are scarcely aware, can be reduced to the form of a liquid, but the
pressure exerted upon the portion to be liquefied must be thirty-nine
times as great as the atmospheric pressure, and the temperature must
have been reduced to a very low point.

93. Artificial Ice. Ammonia gas is liquefied by strong pressure and
low temperature and is then allowed to flow into pipes which run
through tanks containing salt water. The reduction of pressure causes
the liquid to evaporate or turn to a gas, and the fall of temperature
which always accompanies evaporation means a lowering of the
temperature of the salt water to 16° or 18° below zero. But immersed
in the salt water are molds containing pure water, and since the
freezing point of water is 0° C, the water in the molds freezes and
can be drawn from the mold as solid cakes of ice.

[Illustration: FIG. 56.--Apparatus for making artificial ice.]

Ammonia gas is driven by the pump _C_ into the coil _D_ (Fig. 56)
under a pressure strong enough to liquefy it, the heat generated by
this compression being carried off by cold water which constantly
circulates through _B_. The liquid ammonia flows through the
regulating valve _V_ into the coil _E_, in which the pressure is kept
low by the pump _C_. The accompanying expansion reduces the
temperature to a very low degree, and the brine which circulates
around the coil _E_ acquires a temperature below the freezing point of
pure water. The cold brine passes from _A_ to a tank in which are
immersed cans filled with water, and within a short time the water in
the cans is frozen into solid cakes of ice.




CHAPTER IX

INVISIBLE OBJECTS


94. Very Small Objects. We saw in Section 84 that gases have a
tendency to expand, but that they can be compressed by the application
of force. This observation has led scientists to suppose that
substances are composed of very minute particles called molecules,
separated by small spaces called pores; and that when a gas is
condensed, the pores become smaller, and that when a gas expands, the
pores become larger.

The fact that certain substances are soluble, like sugar in water,
shows that the molecules of sugar find a lodging place in the spaces
or pores between the molecules of water, in much the same way that
pebbles find lodgment in the chinks of the coal in a coal scuttle. An
indefinite quantity of sugar cannot be dissolved in a given quantity
of liquid, because after a certain amount of sugar has been dissolved
all the pores become filled, and there is no available molecular
space. The remainder of the sugar settles at the bottom of the vessel,
and cannot be dissolved by any amount of stirring.

If a piece of potassium permanganate about the size of a grain of sand
is put into a quart of water, the solid disappears and the water
becomes a deep rich red. The solid evidently has dissolved and has
broken up into minute particles which are too small to be seen, but
which have scattered themselves and lodged in the pores of the water,
thus giving the water its rich color.

There is no visible proof of the existence of molecules and molecular
spaces, because not only are our eyes unable to see them directly, but
even the most powerful microscope cannot make them visible to us. They
are so small that if one thousand of them were laid side by side, they
would make a speck too small to be seen by the eye and too small to be
visible under the most powerful microscope.

We cannot see molecules or molecular pores, but the phenomena of
compression and expansion, solubility and other equally convincing
facts, have led us to conclude that all substances are composed of
very minute particles or molecules separated by spaces called pores.

95. Journeys Made by Molecules. If a gas jet is turned on and not
lighted, an odor of gas soon becomes perceptible, not only throughout
the room, but in adjacent halls and even in distant rooms. An uncorked
bottle of cologne scents an entire room, the odor of a rose or violet
permeates the atmosphere near and far. These simple everyday
occurrences seem to show that the molecules of a gas must be in a
state of continual and rapid motion. In the case of the cologne, some
molecules must have escaped from the liquid by the process of
evaporation and traveled through the air to the nose. We know that the
molecules of a liquid are in motion and are continually passing into
the air because in time the vessel becomes empty. The only way in
which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really
saying that the molecules are in motion.

From these phenomena and others it is reasonably clear that substances
are composed of molecules, and that molecules are not inert, quiet
particles, but that they are in incessant motion, moving rapidly
hither and thither, sometimes traveling far, sometimes near. Even the
log of wood which lies heavy and motionless on our woodpile is made
up of countless billions of molecules each in rapid incessant motion.
The molecules of solid bodies cannot escape so readily as those of
liquids and gases, and do not travel far. The log lies year after year
in an apparently motionless condition, but if one's eyes were keen
enough, the molecules would be seen moving among themselves, even
though they cannot escape into the surrounding medium and make long
journeys as do the molecules of liquids and gases.

96. The Companions of Molecules. Common sense tells us that a
molecule of water is not the same as a molecule of vinegar; the
molecules of each are extremely small and in rapid motion, but they
differ essentially, otherwise one substance would be like every other
substance. What is it that makes a molecule of water differ from a
molecule of vinegar, and each differ from all other molecules? Strange
to say, a molecule is not a simple object, but is quite complex, being
composed of one or more smaller particles, called atoms, and the
number and kind of atoms in a molecule determine the type of the
molecule, and the type of the molecule determines the substance. For
example, a glass of water is composed of untold millions of molecules,
and each molecule is a company of three still smaller particles, one
of which is called the oxygen atom and two of which are alike in every
particular and are called hydrogen atoms.

97. Simple Molecules. Generally molecules are composed of atoms
which are different in kind. For example, the molecule of water has
two different atoms, the oxygen atom and the hydrogen atoms; alcohol
has three different kinds of atoms, oxygen, hydrogen, and carbon.
Sometimes, however, molecules are composed of a group of atoms all of
which are alike. Now there are but seventy or eighty different kinds
of atoms, and hence there can be but seventy or eighty different
substances whose molecules are composed of atoms which are alike. When
the atoms comprising a molecule are all alike, the substance is called
an element, and is said to be a simple substance. Throughout the
length and breadth of this vast world of ours there are only about
eighty known elements. An element is the simplest substance
conceivable, because it has not been separated into anything simpler.
Water is a compound substance. It can be separated into oxygen and
hydrogen.

Gold, silver, and lead are examples of elements, and water, alcohol,
cider, sand, and marble are complex substances, or compounds, as we
are apt to call them. Everything, no matter what its size or shape or
character, is formed from the various combinations into molecules of a
few simple atoms, of which there exist about eighty known different
kinds. But few of the eighty known elements play an important part in
our everyday life. The elements in which we are most interested are
given in the following table, and the symbols by which they are known
are placed in columns to the right:

|Oxygen |O |Copper |Cu |Phosphorus |P |
|Hydrogen |H |Iodine |I |Potassium |K |
|Carbon |C |Iron |Fe |Silver |Ag |
|Aluminium Al |Lead |Pb |Sodium |Na | |
|Calcium |Ca |Nickel |Ni |Sulphur |S |
|Chlorine |Cl |Nitrogen |N |Tin |Sn |

We have seen in an earlier experiment that twice as much hydrogen as
oxygen can be obtained from water. Two atoms of the element hydrogen
unite with one atom of the element oxygen to make one molecule of
water. In symbols we express this H_2O. A group of symbols, such as
this, expressing a molecule of a compound is called a _formula_. NaCl
is the formula for sodium chloride, which is the chemical name of
common salt.




CHAPTER X

LIGHT


98. What Light Does for Us. Heat keeps us warm, cooks our food,
drives our engines, and in a thousand ways makes life comfortable and
pleasant, but what should we do without light? How many of us could be
happy even though warm and well fed if we were forced to live in the
dark where the sunbeams never flickered, where the shadows never stole
across the floor, and where the soft twilight could not tell us that
the day was done? Heat and light are the two most important physical
factors in life; we cannot say which is the more necessary, because in
the extreme cold or arctic regions man cannot live, and in the dark
places where the light never penetrates man sickens and dies. Both
heat and light are essential to life, and each has its own part to
play in the varied existence of man and plant and animal.

Light enables us to see the world around us, makes the beautiful
colors of the trees and flowers, enables us to read, is essential to
the taking of photographs, gives us our moving pictures and our magic
lanterns, produces the exquisite tints of stained-glass windows, and
brings us the joy of the rainbow. We do not always realize that light
is beneficial, because sometimes it fades our clothing and our
carpets, and burns our skin and makes it sore. But we shall see that
even these apparently harmful effects of light are in reality of great
value in man's constant battle against disease.

99. The Candle. Natural heat and light are furnished by the sun, but
the absence of the sun during the evening makes artificial light
necessary, and even during the day artificial light is needed in
buildings whose structure excludes the natural light of the sun.
Artificial light is furnished by electricity, by gas, by oil in lamps,
and in numerous other ways. Until modern times candles were the main
source of light, and indeed to-day the intensity, or power, of any
light is measured in candle power units, just as length is measured in
yards; for example, an average gas jet gives a 10 candle power light,
or is ten times as bright as a candle; an ordinary incandescent
electric light gives a 16 candle power light, or furnishes sixteen
times as much light as a candle. Very strong large oil lamps can at
times yield a light of 60 candle power, while the large arc lamps
which flash out on the street corners are said to furnish 1200 times
as much light as a single candle. Naturally all candles do not give
the same amount of light, nor are all candles alike in size. The
candles which decorate our tea tables are of wax, while those which
serve for general use are of paraffin and tallow.

[Illustration: FIG. 57.--A photograph at _a_ receives four times as
much light as when held at _b_.]

100. Fading Illumination. The farther we move from a light, the less
strong, or intense, is the illumination which reaches us; the light of
the street lamp on the corner fades and becomes dim before the middle
of the block is reached, so that we look eagerly for the next lamp.
The light diminishes in brightness much more rapidly than we realize,
as the following simple experiment will show. Let a single candle
(Fig. 57) serve as our light, and at a distance of one foot from the
candle place a photograph. In this position the photograph receives a
definite amount of light from the candle and has a certain brightness.

If now we place a similar photograph directly behind the first
photograph and at a distance of two feet from the candle, the second
photograph receives no light because the first one cuts off all the
light. If, however, the first photograph is removed, the light which
fell on it passes outward and spreads itself over a larger area, until
at the distance of the second photograph the light spreads itself over
four times as large an area as formerly. At this distance, then, the
illumination on the second photograph is only one fourth as strong as
it was on a similar photograph held at a distance of one foot from the
candle.

The photograph or object placed at a distance of one foot from a light
is well illuminated; if it is placed at a distance of two feet, the
illumination is only one fourth as strong, and if the object is placed
three feet away, the illumination is only one ninth as strong. This
fact should make us have thought and care in the use of our eyes. We
think we are sixteen times as well off with our incandescent lights as
our ancestors were with simple candles, but we must reflect that our
ancestors kept the candle near them, "at their elbow," so to speak,
while we sit at some distance from the light and unconcernedly read
and sew.

As an object recedes from a light the illumination which it receives
diminishes rapidly, for the strength of the illumination is inversely
proportional to the square of distance of the object from the light.
Our ancestors with a candle at a distance of one foot from a book were
as well off as we are with an incandescent light four feet away.

101. Money Value of Light. Light is bought and sold almost as
readily as are the products of farm and dairy; many factories,
churches, and apartments pay a definite sum for electric light of a
standard strength, and naturally full value is desired. An instrument
for measuring the strength of a light is called a photometer, and
there are many different varieties, just as there are varieties of
scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with
the square of the distance of the object from the light. Suppose we
wish to measure the strength of the electric light bulbs in our homes,
in order to see whether we are getting the specified illumination. In
front of a screen place a black rod (Fig. 58) which is illuminated by
two different lights; namely, a standard candle and an incandescent
bulb whose strength is to be measured. Two shadows of the rod will
fall on the screen, one caused by the candle and the other caused by
the incandescent light. The shadow due to the latter source is not so
dark as that due to the candle. Now let the incandescent light be
moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the
screen as the candle, and the shadows are equal, we know, by Section
100, that its strength is sixteen candle power. If the incandescent
light is four times as far away from the screen as the candle is, its
power must be sixteen times as great, and we know the company is
furnishing the standard amount of light for a sixteen candle power
electric bulb. If, however, the bulb must be moved nearer to the rod
in order that the two shadows may be similar then the light given by
the bulb is less than sixteen candle power, and less than that due the
consumer.

[Illustration: FIG. 58.--The two shadows are equally dark.]

102. How Light Travels. We never expect to see around a corner, and
if we wish to see through pinholes in three separate pieces of
cardboard, we place the cardboards so that the three holes are in a
straight line. When sunlight enters a dark room through a small
opening, the dust particles dancing in the sun show a straight ray. If
a hole is made in a card, and the card is held in front of a light,
the card casts a shadow, in the center of which is a bright spot. The
light, the hole, and the bright spot are all in the same straight
line. These simple observations lead us to think that light travels in
a straight line.

[Illustration: FIG. 59.--The candle cannot be seen unless the three
pinholes are in a strait line.]

We can always tell the direction from which light comes, either by the
shadow cast or by the bright spot formed when an opening occurs in the
opaque object casting the shadow. If the shadow of a tree falls
towards the west, we know the sun must be in the cast; if a bright
spot is on the floor, we can easily locate the light whose rays stream
through an opening and form the bright spot. We know that light
travels in a straight line, and following the path of the beam which
comes to our eyes, we are sure to locate the light.

103. Good and Bad Mirrors. As we walk along the street, we
frequently see ourselves reflected in the shop windows, in polished
metal signboards, in the metal trimmings of wagons and automobiles;
but in mirrors we get the best image of ourselves. We resent the image
given by a piece of tin, because the reflection is distorted and does
not picture us as we really are; a rough surface does not give a fair
representation; if we want a true image of ourselves, we must use a
smooth surface like a mirror as a reflector. If the water in a pond
is absolutely still, we get a clear, true image of the trees, but if
there are ripples on the surface, the reflection is blurred and
distorted. A metal roof reflects so much light that the eyes are
dazzled by it, and a whitewashed fence injures the eyes because of the
glare which comes from the reflected light. Neither of these could be
called mirrors, however, because although they reflect light, they
reflect it so irregularly that not even a suggestion of an image can
be obtained.

Most of us are sufficiently familiar with mirrors to know that the
image is a duplicate of ourselves with regard to size, shape, color,
and expression, but that it appears to be back of the mirror, while we
are actually in front of the mirror. The image appears not only behind
the mirror, but it is also exactly as far back of the mirror as we are
in front of it; if we approach the mirror, the image also draws
nearer; if we withdraw, it likewise recedes.

104. The Path of Light. If a mirror or any other polished surface is
held in the path of a sunbeam, some of the light is reflected, and by
rotating the mirror the reflected sunbeam may be made to take any
path. School children amuse themselves by reflecting sunbeams from a
mirror into their companions' faces. If the companion moves his head
in order to avoid the reflected beam, his tormentor moves or inclines
the mirror and flashes the beam back to his victim's face.

If a mirror is held so that a ray of light strikes it in a
perpendicular direction, the light is reflected backward along the
path by which it came. If, however, the light makes an angle with the
mirror, its direction is changed, and it leaves the mirror along a new
path. By observation we learn that when a beam strikes the mirror and
makes an angle of 30° with the perpendicular, the beam is reflected in
such a way that its new path also makes an angle of 30° with the
perpendicular. If the sunbeam strikes the mirror at an angle of 32°
with the perpendicular, the path of the reflected ray also makes an
angle of 32° with the perpendicular. The ray (_AC_, Fig. 60) which
falls upon the mirror is called the incident ray, and the angle which
the incident ray (_AC_) makes with the perpendicular (_BC_) to the
mirror, at the point where the ray strikes the mirror, is called the
angle of incidence. The angle formed by the reflected ray (_CD_) and
this same perpendicular is called the angle of reflection. Observation
and experiment have taught us that light is always reflected in such a
way that the angle of reflection equals the angle of incidence. Light
is not the only illustration we have of the law of reflection. Every
child who bounces a ball makes use of this law, but he uses it
unconsciously. If an elastic ball is thrown perpendicularly against
the floor, it returns to the sender; if it is thrown against the floor
at an angle (Fig. 61), it rebounds in the opposite direction, but
always in such a way that the angle of reflection equals the angle of
incidence.

[Illustration: FIG. 60.--The ray _AC_ is reflected as _CD_.]

[Illustration: FIG. 61.--A bouncing ball illustrates the law of
reflection.]

105. Why the Image seems to be behind the Mirror. If a candle is
placed in front of a mirror, as in Figure 62, one of the rays of light
which leaves the candle will fall upon the mirror as _AB_ and will be
reflected as _BC_ (in such a way that the angle of reflection equals
the angle of incidence). If an observer stands at _C_, he will think
that the point _A_ of the candle is somewhere along the line _CB_
extended. Such a supposition would be justified from Section 102. But
the candle sends out light in all directions; one ray therefore will
strike the mirror as _AD_ and will be reflected as _DE_, and an
observer at _E_ will think that the point _A_ of the candle is
somewhere along the line _ED_. In order that both observers may be
correct, that is, in order that the light may seem to be in both these
directions, the image of the point _A_ must seem to be at the
intersection of the two lines. In a similar manner it can be shown
that every point of the image of the candle seems to be behind the
mirror.

[Illustration: FIG. 62.--The image is a duplicate of the object, but
appears to be behind the mirror.]

It can be shown by experiment that the distance of the image behind
the mirror is equal to the distance of the object in front of the
mirror.

106. Why Objects are Visible. If the beam of light falls upon a
sheet of paper, or upon a photograph, instead of upon a smooth
polished surface, no definite reflected ray will be seen, but a glare
will be produced by the scattering of the beam of light. The surface
of the paper or photograph is rough, and as a result, it scatters the
beam in every direction. It is hard for us to realize that a smooth
sheet of paper is by no means so smooth as it looks. It is rough
compared with a polished mirror. The law of reflection always holds,
however, no matter what the reflecting surface is,--the angle of
reflection always equals the angle of incidence. In a smooth body the
reflected beams are all parallel; in a rough body, the reflected beams
are inclined to each other in all sorts of ways, and no two beams
leave the paper in exactly the same direction.

[Illustration: FIG. 63.--The surface of the paper, although smooth in
appearance, is in reality rough, and scatters the light in every
direction.]

Hot coals, red-hot stoves, gas flames, and candles shine by their own
light, and are self-luminous. Objects like chairs, tables, carpets,
have no light within themselves and are visible only when they receive
light from a luminous source and reflect that light. We know that
these objects are not self-luminous, because they are not visible at
night unless a lamp or gas is burning. When light from any luminous
object falls upon books, desks, or dishes, it meets rough surfaces,
and hence undergoes diffuse reflection, and is scattered irregularly
in all directions. No matter where the eye is, some reflected rays
enter it, and the various objects are clearly seen.




CHAPTER XI

REFRACTION


107. Bent Rays of Light. A straw in a glass of lemonade seems to be
broken at the surface of the liquid, the handle of a teaspoon in a cup
of water appears broken, and objects seen through a glass of water may
seem distorted and changed in size. When light passes from air into
water, or from any transparent substance into another of different
density, its direction is changed, and it emerges along an entirely
new path (Fig. 64). We know that light rays pass through glass,
because we can see through the window panes and through our
spectacles; we know that light rays pass through water, because we can
see through a glass of clear water; on the other hand, light rays
cannot pass through wood, leather, metal, etc.

[Illustration: FIG. 64.--A straw or stick in water seems broken.]

Whenever light meets a transparent substance obliquely, some of it is
reflected, undergoing a change in its direction; and some of it passes
onward through the medium, but the latter portion passes onward along
a new path. The ray _RO_ (Fig. 65) passes obliquely through the air to
the surface of the water, but, on entering the water, it is bent or
refracted and takes the new path _OS_. The angle _AOR_ is called the
angle of incidence. The angle _POS_ is called the angle of refraction.

[Illustration: FIG. 65.--When the ray _RO_ enters the water, its path
changes to _OS_.]

The angle of refraction is the angle formed by the refracted ray and
the perpendicular to the surface at the point where the light strikes
it.

When light passes from air into water or glass, the refracted ray is
bent toward the perpendicular, so that the angle of refraction is
smaller than the angle of incidence. When a ray of light passes from
water or glass into air, the refracted ray is bent away from the
perpendicular so that the angle of refraction is greater than the
angle of incidence.

The bending or deviation of light in its passage from one substance to
another is called refraction.

108. How Refraction Deceives us. Refraction is the source of many
illusions; bent rays of light make objects appear where they really
are not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of the
stick in Figure 64 seems to be nearer the surface of the water than it
really is.

[Illustration: FIG. 66.--A fish at _A_ seems to be at _B_.]

The light from the sun, moon, and stars can reach us only by passing
through the atmosphere, but in Section 76, we learned that the
atmosphere varies in density from level to level; hence all the light
which travels through the atmosphere is constantly deviated from its
original path, and before the light reaches the eye it has undergone
many changes in direction. Now we learned in Section 102, that the
direction of the rays of light as they enter the eye determines the
direction in which an object is seen; hence the sun, moon, and stars
seem to be along the lines which enter the eye, although in reality
they are not.

109. Uses of Refraction. If it were not for refraction, or the
deviation of light in its passage from medium to medium, the wonders
and beauties of the magic lantern and the camera would be unknown to
us; sun, moon, and stars could not be made to yield up their distant
secrets to us in photographs; the comfort and help of spectacles would
be lacking, spectacles which have helped unfold to many the rare
beauties of nature, such as a clear view of clouds and sunset, of
humming bee and flying bird. Books with their wealth of entertainment
and information would be sealed to a large part of mankind, if glasses
did not assist weak eyes.

By refraction the magnifying glass reveals objects hidden because of
their minuteness, and enlarges for our careful contemplation objects
otherwise barely visible. The watchmaker, unassisted by the magnifying
glass, could not detect the tiny grains of dust or sand which clog the
delicate wheels of our watches. The merchant, with his lens, examines
the separate threads of woolen and silk fabrics to determine the
strength and value of the material. The physician, with his invaluable
microscope, counts the number of infinitesimal corpuscles in the blood
and bases his prescription on that count; he examines the sputum of a
patient to determine whether tuberculosis wastes the system. The
bacteriologist with the same instrument scrutinizes the drinking water
and learns whether the dangerous typhoid germs are present. The
future of medicine will depend somewhat upon the additional secrets
which man is able to force from nature through the use of powerful
lenses, because as lenses have, in the past, been the means of
revealing disease germs, so in the future more powerful lenses may
serve to bring to light germs yet unknown. How refraction accomplishes
these results will be explained in the following Sections.

110. The Window Pane. We have seen that light is bent when it passes
from one medium to another of different density, and that objects
viewed by refracted light do not appear in their proper positions.

When a ray of light passes through a piece of plane glass, such as a
window pane (Fig. 67), it is refracted at the point _B_ toward the
perpendicular, and continues its course through the glass in the new
direction _BC_. On emerging from the glass, the light is refracted
away from the perpendicular and takes the direction _CD_, which is
clearly parallel to its original direction. Hence, when we view
objects through the window, we see them slightly displaced in
position, but otherwise unchanged. The deviation or displacement
caused by glass as thin as window panes is too slight to be noticed,
and we are not conscious that objects are out of position.

[Illustration: FIG. 67.--Objects looked at through a window pane seem
to be in their natural place.]

111. Chandelier Crystals and Prisms. When a ray of light passes
through plane glass, like a window pane, it is shifted somewhat, but
its direction does not change; that is, the emergent ray is parallel
to the incident ray. But when a beam of light passes through a
triangular glass prism, such as a chandelier crystal, its direction is
greatly changed, and an object viewed through a prism is seen quite
out of its true position.

Whenever light passes through a prism, it is bent toward the base of
the prism, or toward the thick portion of the prism, and emerges from
the prism in quite a different direction from that in which it entered
(Fig. 68). Hence, when an object is looked at through a prism, it is
seen quite out of place. In Figure 68, the candle seems to be at _S_,
while in reality it is at _A_.

[Illustration: FIG. 68.--When looked at through the prism, _A_ seems
to be at _S_.]

112. Lenses. If two prisms are arranged as in Figure 69, and two
parallel rays of light fall upon the prisms, the beam _A_ will be bent
downward toward the thickened portion of the prism, and the beam _B_
will be bent upward toward the thick portion of the prism, and after
passing through the prism the two rays will intersect at some point
_F_, called a focus.

[Illustration: FIG. 69.--Rays of light are converged and focused at
_F_.]

If two prisms are arranged as in Figure 70, the ray _A_ will be
refracted upward toward the thick end, and the ray _B_ will be
refracted downward toward the thick end; the two rays, on emerging,
will therefore be widely separated and will not intersect.

[Illustration: FIG. 70.--Rays of light are diverged and do not come to
any real focus.]

Lenses are very similar to prisms; indeed, two prisms placed as in
Figure 69, and rounded off, would make a very good convex lens. A lens
is any transparent material, but usually glass, with one or both sides
curved. The various types of lenses are shown in Figure 71.

[Illustration: FIG. 71.--The different types of lenses.]

The first three types focus parallel rays at some common point _F_, as
in Figure 69. Such lenses are called convex or converging lenses. The
last three types, called concave lenses, scatter parallel rays so that
they do not come to a focus, but diverge widely after passage through
the lens.

113. The Shape and Material of a Lens. The main or principal focus
of a lens, that is, the point at which rays parallel to the base line
_AB_ meet (Fig. 71), depends upon the shape of the lens. For example,
a thick lens, such as _A_ (Fig. 72), focuses the rays very near to the
lens; _B_, which is not so thick, focuses the rays at a greater
distance from the lens; and _C_, which is a very thin lens, focuses
the rays at a considerable distance from the lens. The distance of the
principal focus from the lens is called the focal length of the lens,
and from the diagrams we see that the more convex the lens, the
shorter the focal length.

[Illustration: FIG. 72.--The more curved the lens, the shorter the
focal length, and the nearer the focus is to the lens.]

The position of the principal focus depends not only on the shape of
the lens, but also on the refractive power of the material composing
the lens. A lens made of ice would not deviate the rays of light so
much as a lens of similar shape composed of glass. The greater the
refractive power of the lens, the greater the bending, and the nearer
the principal focus to the lens.

There are many different kinds of glass, and each kind of glass
refracts the light differently. Flint glass contains lead; the lead
makes the glass dense, and gives it great refractive power, enabling
it to bend and separate light in all directions. Cut glass and toilet
articles are made of flint glass because of the brilliant effects
caused by its great refractive power, and imitation gems are commonly
nothing more than polished flint glass.

114. How Lenses Form Images. Suppose we place an arrow, _A_, in
front of a convex lens (Fig. 73). The ray _AC_, parallel to the
principal axis, will pass through the lens and emerge as _DE_. The ray
is always bent toward the thick portion of the lens, both at its
entrance into the lens and its emergence from the lens.

[Illustration: FIG. 73.--The image is larger than the object. By means
of a lens, a watchmaker gets an enlarged image of the dust which clogs
the wheels of his watch.]

In Section 105, we saw that two rays determine the position of any
point of our image; hence in order to locate the image of the top of
the arrow, we need to consider but one more ray from the top of the
object. The most convenient ray to choose would be one passing through
_O_, the optical center of the lens, because such a ray passes through
the lens unchanged in direction, as is clear from Figure 74. The point
where _AC_ and _AO_ meet after refraction will be the position of the
top of the arrow. Similarly it can be shown that the center of the
arrow will be at the point _T_, and we see that the image is larger
than the object. This can be easily proved experimentally. Let a
convex lens be placed near a candle (Fig. 75); move a paper screen
back and forth behind the lens; for some position of the screen a
clear, enlarged image of the candle will be made.

[Illustration: FIG. 74.--Rays above _O_ are bent downward, those below
_O_ are bent upward, and rays through _O_ emerge from the lens
unchanged in direction.]

If the candle or arrow is placed in a new position, say at _MA_ (Fig.
76), the image formed is smaller than the object, and is nearer to the
lens than it was before. Move the lens so that its distance from the
candle is increased, and then find the image on a piece of paper. The
size and position of the image depend upon the distance of the object
from the lens (Fig. _77_). By means of a lens one can easily get on a
visiting card a picture of a distant church steeple.

[Illustration: FIG. 75.--The lens is held in such a position that the
image of the candle is larger than the object.]

[Illustration: FIG. 76.--The image is smaller than the object.]

115. The Value of Lenses. If it were not for the fact that a lens
can be held at such a distance from an object as to make the image
larger than the object, it would be impossible for the lens to assist
the watchmaker in locating the small particles of dust which clog the
wheels of the watch. If it were not for the opposite fact--that a lens
can be held at such a distance from the object as to make an image
smaller than the object, it would be impossible to have a photograph
of a tall tree or building unless the photograph were as large as the
tree itself. When a photographer takes a photograph of a person or a
tree, he moves his camera until the image formed by the lens is of the
desired size. By bringing the camera (really the lens of the camera)
near, we obtain a large-sized photograph; by increasing the distance
between the camera and the object, a smaller photograph is obtained.
The mountain top may be so far distant that in the photograph it will
not appear to be greater than a small stone.

[Illustration: FIG. 77.--The lens is placed in such a position that
the image is about the same size as the object.]

Many familiar illustrations of lenses, or curved refracting surfaces,
and their work, are known to all of us. Fish globes magnify the fish
that swim within. Bottles can be so shaped that they make the olives,
pickles, and peaches that they contain appear larger than they really
are. The fruit in bottles frequently seems too large to have gone
through the neck of the bottle. The deception is due to refraction,
and the material and shape of the bottle furnish a sufficient
explanation.

By using combinations of two or more lenses of various kinds, it is
possible to have an image of almost any desired size, and in
practically any desired position.

116. The Human Eye. In Section 114, we obtained on a movable screen,
by means of a simple lens, an image of a candle. The human eye
possesses a most wonderful lens and screen (Fig. 78); the lens is
called the crystalline lens, and the screen is called the retina. Rays
of light pass from the object through the pupil _P_, go through the
crystalline lens _L_, where they are refracted, and then pass onward
to the retina _R_, where they form a distinct image of the object.

[Illustration: FIG. 78.--The eye.]

We learned in Section 114 that a change in the position of the object
necessitated a change in the position of the screen, and that every
time the object was moved the position of the screen had to be altered
before a clear image of the object could be obtained. The retina of
the eye cannot be moved backward and forward, as the screen was, and
the crystalline lens is permanently located directly back of the iris.
How, then, does it happen that we can see clearly both near and
distant objects; that the printed page which is held in the hand is
visible at one second, and that the church spire on the distant
horizon is visible the instant the eyes are raised from the book? How
is it possible to obtain on an immovable screen by means of a simple
lens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changes
shape according to need. The lens is attached to the eye by means of
small muscles, _m_, and it is by the action of these muscles that the
lens is able to become small and thick, or large and thin; that is, to
become more or less curved. When we look at near objects, the muscles
act in such a way that the lens bulges out, and becomes thick in the
middle and of the right curvature to focus the near object upon the
screen. When we look at an object several hundred feet away, the
muscles change their pull on the lens and flatten it until it is of
the proper curvature for the new distance. The adjustment of the
muscles is so quick and unconscious that we normally do not experience
any difficulty in changing our range of view. The ability of the eye
to adjust itself to varying distances is called accommodation. The
power of adjustment in general decreases with age.

117. Farsightedness and Nearsightedness. A farsighted person is one
who cannot see near objects so distinctly as far objects, and who in
many cases cannot see near objects at all. The eyeball of a farsighted
person is very short, and the retina is too close to the crystalline
lens. Near objects are brought to a focus behind the retina instead of
on it, and hence are not visible. Even though the muscles of
accommodation do their best to bulge and thicken the lens, the rays of
light are not bent sufficiently to focus sharply on the retina. In
consequence objects look blurred. Farsightedness can be remedied by
convex glasses, since they bend the light and bring it to a closer
focus. Convex glasses, by bending the rays and bringing them to a
nearer focus, overbalance a short eyeball with its tendency to focus
objects behind the retina.

[Illustration: FIG. 79.--The farsighted eye.]

[Illustration: FIG. 80.--The defect is remedied by convex glasses.]

A nearsighted person is one who cannot see objects unless they are
close to the eye. The eyeball of a nearsighted person is very wide,
and the retina is too far away from the crystalline lens. Far objects
are brought to a focus in front of the retina instead of on it, and
hence are not visible. Even though the muscles of accommodation do
their best to pull out and flatten the lens, the rays are not
separated sufficiently to focus as far back as the retina. In
consequence objects look blurred. Nearsightedness can be remedied by
wearing concave glasses, since they separate the light and move the
focus farther away. Concave glasses, by separating the rays and making
the focus more distant, overbalance a wide eyeball with its tendency
to focus objects in front of the retina.

[Illustration: FIG. 81.--The nearsighted eye. The defect is remedied
by concave glasses.]

118. Headache and Eyes. Ordinarily the muscles of accommodation
adjust themselves easily and quickly; if, however, they do not,
frequent and severe headaches occur as a result of too great muscular
effort toward accommodation. Among young people headaches are
frequently caused by over-exertion of the crystalline muscles. Glasses
relieve the muscles of the extra adjustment, and hence are effective
in eliminating this cause of headache.

An exact balance is required between glasses, crystalline lens, and
muscular activity, and only those who have studied the subject
carefully are competent to treat so sensitive and necessary a part of
the body as the eye. The least mistake in the curvature of the
glasses, the least flaw in the type of glass (for example, the kind of
glass used), means an improper focus, increased duty for the muscles,
and gradual weakening of the entire eye, followed by headache and
general physical discomfort.

119. Eye Strain. The extra work which is thrown upon the nervous
system through seeing, reading, writing, and sewing with defective
eyes is recognized by all physicians as an important cause of disease.
The tax made upon the nervous system by the defective eye lessens the
supply of energy available for other bodily use, and the general
health suffers. The health is improved when proper glasses are
prescribed.

Possibly the greatest danger of eye strain is among school children,
who are not experienced enough to recognize defects in sight. For this
reason, many schools employ a physician who examines the pupils' eyes
at regular intervals.

The following general precautions are worth observing:--

1. Rest the eyes when they hurt, and as far as possible do close work,
such as writing, reading, sewing, wood carving, etc., by daylight.

2. Never read in a very bright or a very dim light.

3. If the light is near, have it shaded.

4. Do not rub the eyes with the fingers.

5. If eyes are weak, bathe them in lukewarm water in which a pinch of
borax has been dissolved.




CHAPTER XII

PHOTOGRAPHY


120. The Magic of the Sun. Ribbons and dresses washed and hung in
the sun fade; when washed and hung in the shade, they are not so apt
to lose their color. Clothes are laid away in drawers and hung in
closets not only for protection against dust, but also against the
well-known power of light to weaken color.

Many housewives lower the window shades that the wall paper may not
lose its brilliancy, that the beautiful hues of velvet, satin, and
plush tapestry may not be marred by loss in brilliancy and sheen.
Bright carpets and rugs are sometimes bought in preference to more
delicately tinted ones, because the purchaser knows that the latter
will fade quickly if used in a sunny room, and will soon acquire a
dull mellow tone. The bright and gay colors and the dull and somber
colors are all affected by the sun, but why one should be affected
more than another we do not know. Thousands of brilliant and dainty
hues catch our eye in the shop and on the street, but not one of them
is absolutely permanent; some may last for years, but there is always
more or less fading in time.

Sunlight causes many strange, unexplained effects. If the two
substances, chlorine and hydrogen, are mixed in a dark room, nothing
remarkable occurs any more than though water and milk were mixed, but
if a mixture of these substances is exposed to sunlight, a violent
explosion occurs and an entirely new substance is formed, a compound
entirely different in character from either of its components.

By some power not understood by man, the sun is able to form new
substances. In the dark, chlorine and hydrogen are simply chlorine and
hydrogen; in the sunlight they combine as if by magic into a totally
different substance. By the same unexplained power, the sun frequently
does just the opposite work; instead of combining two substances to
make one new product, the sun may separate or break down some
particular substance into its various elements. For example, if the
sun's rays fall upon silver chloride, a chemical action immediately
begins, and as a result we have two separate substances, chlorine and
silver. The sunlight separates silver chloride into its constituents,
silver and chlorine.

121. The Magic Wand in Photography. Suppose we coat one side of a
glass plate with silver chloride, just as we might put a coat of
varnish on a chair. We must be very careful to coat the plate in the
dark room,[B] otherwise the sunlight will separate the silver chloride
and spoil our plan. Then lay a horseshoe on the plate for good luck,
and carry the plate out into the light for a second. The light will
separate the silver chloride into chlorine and silver, the latter of
which will remain on the plate as a thin film. All of the plate was
affected by the sun except the portion protected by the horseshoe
which, because it is opaque, would not allow light to pass through and
reach the plate. If now the plate is carried back to the dark room and
the horseshoe is removed, one would expect to see on the plate an
impression of the horseshoe, because the portion protected by the
horseshoe would be covered by silver chloride and the exposed
unprotected portion would be covered by metallic silver. But we are
much disappointed because the plate, when examined ever so carefully,
shows not the slightest change in appearance. The change is there, but
the unaided eye cannot detect the change. Some chemical, the
so-called "developer," must be used to bring out the hidden change and
to reveal the image to our unseeing eyes. There are many different
developers in use, any one of which will effect the necessary
transformation. When the plate has been in the developer for a few
seconds, the silver coating gradually darkens, and slowly but surely
the image printed by the sun's rays appears. But we must not take this
picture into the light, because the silver chloride which was
protected by the horseshoe is still present, and would be strongly
affected by the first glimmer of light, and, as a result, our entire
plate would become similar in character and there would be no contrast
to give an image of the horseshoe on the plate.

[Footnote B: That is, a room from which ordinary daylight is
excluded.]

But a photograph on glass, which must be carefully shielded from the
light and admired only in the dark room, would be neither pleasurable
nor practical. If there were some way by which the hitherto unaffected
silver chloride could be totally removed, it would be possible to take
the plate into any light without fear. To accomplish this, the
unchanged silver chloride is got rid of by the process technically
called "fixing"; that is, by washing off the unreduced silver chloride
with a solution such as sodium thiosulphite, commonly known as hypo.
After a bath in the hypo the plate is cleansed in clear running water
and left to dry. Such a process gives a clear and permanent picture on
the plate.

[Illustration: FIG. 82.--A camera.]

122. The Camera. A camera (Fig. 82) is a light-tight box containing
a movable convex lens at one end and a screen at the opposite end.
Light from the object to be photographed passes through the lens,
falls upon the screen, and forms an image there. If we substitute for
the ordinary screen a plate or film coated with silver chloride or any
other silver salt, the light which falls upon the sensitive plate and
forms an image there will change the silver chloride and produce a
hidden image. If the plate is then removed from the camera in the
dark, and is treated as described in the preceding Section, the image
becomes visible and permanent. In practice some gelatin is mixed with
the silver salt, and the mixture is then poured over the plate or film
in such a way that a thin, even coating is made. It is the presence of
the gelatin that gives plates a yellowish hue. The sensitive plates
are left to dry in dark rooms, and when the coating has become
absolutely firm and dry, the plates are packed in boxes and sent forth
for sale.

Glass plates are heavy and inconvenient to carry, so that celluloid
films have almost entirely taken their place, at least for outdoor
work.

123. Light and Shade. Let us apply the above process to a real
photograph. Suppose we wish to take the photograph of a man sitting in
a chair in his library. If the man wore a gray coat, a black tie, and
a white collar, these details must be faithfully represented in the
photograph. How can the almost innumerable lights and shades be
produced on the plate?

The white collar would send through the lens the most light to the
sensitive plate; hence the silver chloride on the plate would be most
changed at the place where the lens formed an image of the collar. The
gray coat would not send to the lens so much light as the white
collar, hence the silver chloride would be less affected by the light
from the coat than by that from the collar, and at the place where the
lens produced an image of the coat the silver chloride would not be
changed so much as where the collar image is. The light from the face
would produce a still different effect, since the light from the face
is stronger than the light from the gray coat, but less than that from
a white collar. The face in the image would show less changed silver
chloride than the collar, but more than the coat, because the face is
lighter than the coat, but not so light as the collar. Finally, the
silver chloride would be least affected by the dark tie. The wall
paper in the background would affect the plate according to the
brightness of the light which fell directly upon it and which
reflected to the camera. When such a plate has been developed and
fixed, as described in Section 121, we have the so-called negative
(Fig. 83). The collar is very dark, the black tie and gray coat white,
and the white tidy very dark.

[Illustration: FIG. 83.--A negative.]

The lighter the object, such as tidy or collar, the more salt is
changed, or, in other words, the greater the portion of the silver
salt that is affected, and hence the darker the stain on the plate at
that particular spot. The plate shows all gradations of intensity--the
tidy is dark, the black tie is light. The photograph is true as far as
position, form, and expression are concerned, but the actual
intensities are just reversed. How this plate can be transformed into
a photograph true in every detail will be seen in the following
Section.

124. The Perfect Photograph. Bright objects, such as the sky or a
white waist, change much of the silver chloride, and hence appear
dark on the negative. Dark objects, such as furniture or a black coat,
change little of the chloride, and hence appear light on the negative.
To obtain a true photograph, the negative is placed on a piece of
sensitive photographic paper, or paper coated with a silver salt in
the same manner as the plate and films. The combination is exposed to
the light. The dark portions of the negative will act as obstructions
to the passage of light, and but little light will pass through that
part of the negative to the photographic paper, and consequently but
little of the silver salt on the paper will be changed. On the other
hand, the light portion of the negative will allow free and easy
passage of the light rays, which will fall upon the photographic paper
and will change much more of the silver. Thus it is that dark places
in the negative produce light places in the positive or real
photograph (Fig. 84), and that light places in the negative produce
dark places in the positive; all intermediate grades are likewise
represented with their proper gradations of intensity.

[Illustration: FIG. 84.--A positive or true photograph.]

If properly treated, a negative remains good for years, and will serve
for an indefinite number of positives or true photographs.

125. Light and Disease. The far-reaching effect which light has upon
some inanimate objects, such as photographic films and clothes, leads
us to inquire into the relation which exists between light and living
things. We know from daily observation that plants must have light in
order to thrive and grow. A healthy plant brought into a dark room
soon loses its vigor and freshness, and becomes yellow and drooping.
Plants do not all agree as to the amount of light they require, for
some, like the violet and the arbutus, grow best in moderate light,
while others, like the willows, need the strong, full beams of the
sun. But nearly all common plants, whatever they are, sicken and die
if deprived of sunlight for a long time. This is likewise true in the
animal world. During long transportation, animals are sometimes
necessarily confined in dark cars, with the result that many deaths
occur, even though the car is well aired and ventilated and the food
supply good. Light and fresh air put color into pale cheeks, just as
light and air transform sickly, yellowish plants into hardy green
ones. Plenty of fresh air, light, and pure water are the watchwords
against disease.

[Illustration: FIG. 85--Stems and leaves of oxalis growing toward the
light.]

In addition to the plants and animals which we see, there are many
strange unseen ones floating in the atmosphere around us, lying in the
dust of corner and closet, growing in the water we drink, and
thronging decayed vegetable and animal matter. Everyone knows that
mildew and vermin do damage in the home and in the field, but very few
understand that, in addition to these visible enemies of man, there
are swarms of invisible plants and animals some of which do far more
damage, both directly and indirectly, than the seen and familiar
enemies. All such very small plants and animals are known as
_microorganisms_.

Not all microörganisms are harmful; some are our friends and are as
helpful to us as are cultivated plants and domesticated animals. Among
the most important of the microörganisms are bacteria, which include
among their number both friend and foe. In the household, bacteria are
a fruitful source of trouble, but some of them are distinctly friends.
The delicate flavor of butter and the sharp but pleasing taste of
cheese are produced by bacteria. On the other hand, bacteria are the
cause of many of the most dangerous diseases, such as typhoid fever,
tuberculosis, influenza, and la grippe.

By careful observation and experimentation it has been shown
conclusively that sunlight rapidly kills bacteria, and that it is only
in dampness and darkness that bacteria thrive and multiply. Although
sunlight is essential to the growth of most plants and animals, it
retards and prevents the growth of bacteria. Dirt and dust exposed to
the sunlight lose their living bacteria, while in damp cellars and
dark corners the bacteria thrive, increasing steadily in number. For
this reason our houses should be kept light and airy; blinds should be
raised, even if carpets do fade; it is better that carpets and
furniture should fade than that disease-producing bacteria should find
a permanent abode within our dwellings. Kitchens and pantries in
particular should be thoroughly lighted. Bedclothes, rugs, and
clothing should be exposed to the sunlight as frequently as possible;
there is no better safeguard against bacterial disease than light. In
a sick room sunlight is especially valuable, because it not only kills
bacteria, but keeps the air dry, and new bacteria cannot get a start
in a dry atmosphere.




CHAPTER XIII

COLOR


126. The Rainbow. One of the most beautiful and well-known phenomena
in nature is the rainbow, and from time immemorial it has been
considered Jehovah's signal to mankind that the storm is over and that
the sunshine will remain. Practically everyone knows that a rainbow
can be seen only when the sun's rays shine upon a mist of tiny drops
of water. It is these tiny drops which by their refraction and their
scattering of light produce the rainbow in the heavens.

The exquisite tints of the rainbow can be seen if we look at an object
through a prism or chandelier crystal, and a very simple experiment
enables us to produce on the wall of a room the exact colors of the
rainbow in all their beauty.

[Illustration: FIG. 86.--White light is a mixture of lights of rainbow
colors.]

127. How to produce Rainbow Colors. _The Spectrum._ If a beam of
sunlight is admitted into a dark room through a narrow opening in the
shade, and is allowed to fall upon a prism, as shown in Figure 86, a
beautiful band of colors will appear on the opposite wall of the room.
The ray of light which entered the room as ordinary sunlight has not
only been refracted and bent from its straight path, but it has been
spread out into a band of colors similar to those of the rainbow.

Whenever light passes through a prism or lens, it is dispersed or
separated into all the colors which it contains, and a band of colors
produced in this way is called a spectrum. If we examine such a
spectrum we find the following colors in order, each color
imperceptibly fading into the next: violet, indigo, blue, green,
yellow, orange, red.

128. Sunlight or White Light. White light or sunlight can be
dispersed or separated into the primary colors or rainbow hues, as
shown in the preceding Section. What seems even more wonderful is that
these spectral colors can be recombined so as to make white light.

If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed
behind _A_ in a reversed position, it will undo the dispersion of _A_,
bending upward the seven different beams in such a way that they
emerge together and produce a white spot on the screen. Thus we see,
from two simple experiments, that all the colors of the rainbow may be
obtained from white light, and that these colors may be in turn
recombined to produce white light.

[Illustration: FIG. 87.--Rainbow colors recombined to form white
light.]

White light is not a simple light, but is composed of all the colors
which appear in the rainbow.

129. Color. If a piece of red glass is held in the path of the
colored beam of light formed as in Section 127, all the colors on the
wall will disappear except the red, and instead of a beautiful
spectrum of all colors there will be seen the red color alone. The red
glass does not allow the passage through it of any light except red
light; all other colors are absorbed by the red glass and do not reach
the eye. Only the red ray passes through the red glass, reaches the
eye, and produces a sensation of color.

If a piece of blue glass is substituted for the red glass, the blue
band remains on the wall, while all the other colors disappear. If
both blue and red pieces of glass are held in the path of the beam, so
that the light must pass through first one and then the other, the
entire spectrum disappears and no color remains. The blue glass
absorbs the various rays with the exception of the blue ones, and the
red glass will not allow these blue rays to pass through it; hence no
light is allowed passage to the eye.

An emerald looks green because it freely transmits green, but absorbs
the other colors of which ordinary daylight is composed. A diamond
appears white because it allows the passage through it of all the
various rays; this is likewise true of water and window panes.

Stained-glass windows owe their charm and beauty to the presence in
the glass of various dyes and pigments which absorb in different
amounts some colors from white light and transmit others. These
pigments or dyes are added to the glass while it is in the molten
state, and the beauty of a stained-glass window depends largely upon
the richness and the delicacy of the pigments used.

130. Reflected Light. _Opaque Objects._ In Section 106 we learned
that most objects are visible to us because of the light diffusely
reflected from them. A white object, such as a sheet of paper, a
whitewashed fence, or a table cloth, absorbs little of the light which
falls upon it, but reflects nearly all, thus producing the sensation
of white. A red carpet absorbs the light rays incident upon it except
the red rays, and these it reflects to the eye.

Any substance or object which reflects none of the rays which fall
upon it, but absorbs all, appears black; no rays reach the eye, and
there is an absence of any color sensation. Coal and tar and soot are
good illustrations of objects which absorb all the light which falls
upon them.

131. How and Why Colors Change. _Matching Colors._ Most women prefer
to shop in the morning and early afternoon when the sunlight
illuminates shops and factories, and when gas and electricity do not
throw their spell over colors. Practically all people know that
ribbons and ties, trimmings and dresses, frequently look different at
night from what they do in the daytime. It is not safe to match colors
by artificial light; cloth which looks red by night may be almost
purple by day. Indeed, the color of an object depends upon the color
of the light which falls upon it. Strange sights are seen on the
Fourth of July when variously colored fireworks are blazing. The child
with a white blouse appears first red, then blue, then green,
according as his powders burn red, blue, or green. The face of the
child changes from its normal healthy hue to a brilliant red and then
to ghastly shades.

Suppose, for example, that a white hat is held at the red end of the
spectrum or in any red light. The characteristics of white objects is
their ability to reflect _all_ the various rays that fall upon them.
Here, however, the only light which falls upon the white hat is red
light, hence the only light which the hat has to reflect is red light
and the hat consequently appears red. Similarly, if a white hat is
placed in a blue light, it will reflect all the light which falls upon
it, namely, blue light, and will appear blue. If a red hat is held in
a red light, it is seen in its proper color. If a red hat is held in a
blue light, it appears black; it cannot reflect any of the blue light
because that is all absorbed and there is no red light to reflect.

A child wearing a green frock on Independence Day seems at night to be
wearing a black frock, if standing near powders burning with red,
blue, or violet light.

132. Pure, Simple Colors--Things as they Seem. To the eye white
light appears a simple, single color. It reveals its compound nature
to us only when passed through a prism, when it shows itself to be
compounded of an infinite number of colors which Sir Isaac Newton
grouped in seven divisions: violet, indigo, blue, green, yellow,
orange, and red.

We naturally ask ourselves whether these colors which compose white
light are themselves in turn compound? To answer that question, let us
very carefully insert a second prism in the path of the rays which
issue from the first prism, carefully barring out the remaining six
kinds of rays. If the red light is compound, it will be broken up into
its constituent parts and will form a typical spectrum of its own,
just as white light did after its passage through a prism. But the red
rays pass through the second prism, are refracted, and bent from this
course, and no new colors appear, no new spectrum is formed. Evidently
a ray of spectrum red is a simple color, not a compound color.

If a similar experiment is made with the remaining spectrum rays, the
result is always the same: the individual spectrum colors remain
simple, pure colors. _The individual spectrum colors are groups of
simple, pure colors._

[Illustration: FIG. 88.--Violet and green give blue. Green, blue, and
red give white.]

133. Colors not as they Seem--Compound Colors. If one half of a
cardboard disk (Fig. 88) is painted green, and the other half violet,
and the disk is slipped upon a toy top, and spun rapidly, the rotating
disk will appear blue; if red and green are used in the same way
instead of green and violet, the rotating disk will appear yellow. A
combination of red and yellow will give orange. The colors formed in
this way do not appear to the eye different from the spectrum colors,
but they are actually very different. The spectrum colors, as we saw
in the preceding Section, are pure, simple colors, while the colors
formed from the rotating disk are in reality compounded of several
totally different rays, although in appearance the resulting colors
are pure and simple.

If it were not that colors can be compounded, we should be limited in
hue and shade to the seven spectral colors; the wealth