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.