Page:Elementary Text-book of Physics (Anthony, 1897).djvu/202

188 boiling-point by the friction of a blunt boring-tool within the bore of a cannon. He showed that the heat manifested in this experiment could not have come from any of the bodies present, and also that heat would continue to be developed as long as the borer continued to revolve, or that the supply of heat was practically inexhaustible. The heat, therefore, must have been generated by the friction.

That ice is not melted by the combination with it of a heat substance was shown early in the present century by Davy. He caused ice to melt by friction of one piece upon another in a vacuum, the experiment being performed in a room where the temperature was below the melting-point of ice. There was no source from which heat could be drawn. The ice must, therefore, have been melted by the friction.

Rumford was convinced that the heat obtained in his experiment was only transformed mechanical energy; but to demonstrate this it was necessary to prove that the quantity of heat produced was always proportional to the quantity of mechanical work done. This was done in the most complete manner by Joule in a series of experiments extending from 1842 to 1849. He showed that, however the heat was produced by mechanical means, whether by the agitation of water by a paddle-wheel, the agitation of mercury, or the friction of iron plates upon each other, the same expenditure of mechanical energy always developed the same quantity of heat. Joule also proved the perfect equivalence of heat and electrical energy.

These experiments prove that heat is a form of energy. Consistent explanations of most if not all of the phenomena of heat may be given if we assume that the molecules of bodies, and the atoms constituting the molecules, are in constant motion, that the temperature of a body varies with the mean kinetic energy of an atom, and that the heat in a body is the sum of the kinetic energies of its atoms.