Page:Elementary Text-book of Physics (Anthony, 1897).djvu/207

§ 166] gram-degree. It is one one-thousandth of the larger unit. It may, therefore, be called a millicalorie.

The fact that heat is energy enables us to employ still another unit. It is that quantity of heat which is equivalent to an erg. This unit is called the mechanical unit of heat. According to the determination of Griffiths, a calorie contains about 41,983,000,000 mechanical units.

165. Heat required to raise the Temperature of a Mass of Water.—It is evident that to raise the temperature of $$m$$ kilograms of water from zero to one degree will require $$m$$ calories. If the temperature of the same quantity of water fall from one degree to zero, the same quantity of heat is given to surrounding bodies.

Experiment shows, that if the same quantity of water be raised to different temperatures, quantities of heat nearly proportional to the rise in temperature will be required: hence, to raise the temperature of $$m$$ kilograms of water from zero to $$t$$ degrees requires $$mt$$ calories very nearly. This is shown by mixing water at a lower temperature with water at a higher temperature. The temperature of the mixture will be almost exactly the mean of the two. Regnault, who tried this experiment with the greatest care, found the temperature of the mixture a little higher than the mean, and concluded that the quantity of heat required to raise the temperature of a kilogram of water one degree increases slightly with the temperature; that is, to raise the temperature of a kilogram of water from twenty to twenty-one degrees, requires a little more heat than to raise the temperature of the same quantity of water from zero to one degree. Rowland found, by mixing water at various temperatures, and also by measuring the energy required to raise the temperature of water by agitation by a paddle-wheel, that, when the air thermometer is taken as a standard, the quantity of heat necessary to raise the temperature of a given quantity of water one degree diminishes slightly from zero to thirty degrees, and then increases to the boiling-point.

166. Specific Heat.—Only one thirtieth as much heat is required to raise the temperature of a kilogram of mercury from zero to one