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