Page:Elementary Text-book of Physics (Anthony, 1897).djvu/201

§ 159] which they contain. This fact shows that the heat separates the metals from their combinations.

When the junction of two dissimilar metals in a conducting circuit is heated, electric currents are produced.

Heat performs mechanical work. For example, the heat produced in the furnace of a steam-boiler may be used to drive an engine.

158. Production of Heat.—Heat is produced by various processes, some of which are the reverse of the operations just mentioned as the effects of heat. As examples of such reverse operations may be mentioned, the production of heat by the compression of a body which expands when heated; the production of heat during a change in the state of aggregation of a body, when the freedom of motion among the molecules is diminished; the production of heat during chemical combination; and the production of heat when an electric current passes through a junction of two dissimilar metals in an opposite direction to that of the current which is set up when the junction is heated.

Heat is produced in general in any process involving the expenditure of mechanical energy. The heat produced in such processes cannot be used to restore the whole of the original mechanical energy. The production of heat by friction is the best example of these processes.

Further, an electric current, in a homogeneous conductor, generates heat at every point in it, while if every point in the conductor be equally heated no current will be set up.

These cases are examples of the production of heat by non-reversible processes.

159. Nature of Heat.—Heat was formerly considered to be a substance which passed from one body to another, lowering the temperature of the one and raising that of the other, which combined with solids to form liquids, and with liquids to form gases or vapors. But the most delicate balances fail to show any change of weight wheh heat passes from one body to another. Rumford was able to raise a considerable quantity of water to the