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CHAP. III.] heat, as the eye distinguishes degrees of light, or the ear of sound.

Heat is a force which is found everywhere, and exists in every body, whether animate or inanimate. It may even be obtained from two pieces of ice by friction, which we know melts them. Cold is simply a relative term, which has no meaning in itself, but is used to signify low degrees of heat; thus, if we say, "Ice is very cold," we imply, "Ice has very little heat."

That every object must possess a certain degree of heat is evident if we regard heat as a mode of motion among particles, for everything in nature is composed of groups of particles. Tyndall, in his great work on "Heat as a Mode of Motion," explains this difficult question very clearly. "It seems possible," he says,1 "to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity, and through the greatest space. . . . Temperature may be conceived to depend upon the velocity of the vibrations; increase of capacity in the motion being performed in greater space." Thus friction produces increase of heat, by increasing the rapidity of the motion of particles, and every case of combustion "may be ascribed to the collision of atoms which have been urged together by their mutual attractions." The particles of all substances have a tendency