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
[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
[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
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
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
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
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
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
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
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
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
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
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.