Page:Elementary Text-book of Physics (Anthony, 1897).djvu/243

§ 221] of saturated vapor at the pressure chosen, and then convert it wholly into steam. The result of his experiments give, for the heat equivalent of vaporization of water at 100°, 537 calories. That is, he found that by condensing a kilogram of steam at 100° into water, and then cooling the water to zero, 637 calories were obtained. But almost exactly 100 calories are derived from the water cooling from 100° to zero; hence 537 calories is the heat equivalent of vaporization at 100°.

219. Dissociation.—It has already been noted (§ 157), that, at high temperatures, compounds are separated into their elements. To effect this separation, the powerful forces of chemical affinity must be overcome, and a considerable amount of energy must be consumed.

220. Heat Equivalent of Dissociation and Chemical Union.—From the principle of the conservation of energy, it may be assumed that the energy required for dissociation is the same as that developed by the reunion of the elements. The heat equivalent of chemical union is not easy to determine, because the process is usually complicated by changes of physical state. We may cause the union of carbon and oxygen in a calorimeter, and, bringing the products of combustion to the temperature of the elements before the union, measure the heat given to the instrument; but the carbon has changed its state from a solid to a gas, and some of the chemical energy must have been consumed in that process. The heat measured is the available heat. The best determinations of the available heat of chemical union have been made by Andrews, Favre and Silbermann, and Berthelot.

'''221. Molecular Motion. States of Matter.'''—The continued production of heat by the expenditure of mechanical work proves that heat is not a substance, and suggests that it must be in some way dependent on motion. It has been seen that such phenomena as expansion and fusion may be explained on the hypothesis that the molecules of a body move more rapidly when the body is heated.