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. 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.