=177. Specific Heat.=--In the study of density and specific gravity it is made clear that different substances differ widely in the amount of matter contained in equal volumes, _e.g._, lead is much denser than water. The study of the relative densities of substance is usually considered under the subject of _specific gravity_. _Specific heat_ as distinguished from specific gravity is concerned with the _capacity_ for heat possessed by different substances. The definition for specific heat is: _The ratio of the amount of heat required to change the temperature of a given mass of a substance 1 C. degree to the amount of heat required to change the temperature of the same mass of water 1 C. degree._ By definition, it requires 1 calorie to raise the temperature of the gram of water 1°C. The _specific heat_ therefore of water is taken as one. The specific heat of most substances except hydrogen, is _less_ than that of water, and as a rule, the denser the body the less its specific heat, as may be observed in the following table: ---------+----------+---------- | Specific | Specific | gravity | heat ---------+----------+---------- Gold | 19.3 | 0.032 Mercury | 13.6 | 0.033 Copper | 8.9 | 0.093 Brass | 8.4-8.9 | 0.094 Nickel | 8.57 | 0.11 Iron | 7.5+ | 0.1125 Aluminum | 2.67 | 0.218 Glass | 2.5-3.6 | 0.19 Ice | 0.918 | 0.504 Air | 0.00129 | 0.237 Steam | 0.00061 | 0.480 Hydrogen | 0.00009 | 3.409 ---------+----------+---------- =178. Method of Determining Specific Heat.=--The specific heat of a body is usually determined by what is called the _method of mixtures_. For example, a definite weight of a substance, say a 200-g. iron ball, is placed in boiling water until it has the temperature of the hot water, 100°C. Suppose that 300 g. of water at 18°C. be placed in a calorimeter, and that the hot iron ball on being placed in the water raises its temperature to 23.5°C. The heat received by the water equals 5.5 × 300 = 1650 calories. This must have come from the heated iron ball. 200 g. of iron then in cooling 76.5°C. (100°-23.5°) gave out 1650 calories. Then 1 g. of iron in cooling 76.5°C. Would give out 8.25 calories or 1 g. of iron cooling 1°C. would yield about 0.11 calorie. The specific heat of the iron is then 0.11. For accurate determination the heat received by the calorimeter must be considered.