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 deg.  C. It is the amount of heat given out by 1 gram of water when its temperature falls 1 deg.  C., or the amount of heat absorbed by 1 gram of water when its temperature rises 1 deg.  C. If 400 grams of water are heated from 0 deg. to 5 deg.  C., the amount of heat which has entered the water is equivalent to 5 x 400 or 2000 calories; if 200 grams of water cool from 25 deg. to 20 deg.  C., the heat given out by the water is equivalent to 5 x 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 deg. than is required to raise the temperature of an equal quantity of water 1 deg..  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 deg. as to heat the same quantity of water 1 deg..

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 deg.  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 deg.  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.