The most general effect of heat is the expansion of matter; solids, liquids, and airs, all expand under its influence. If a bar of metal is exposed to calorific action, it increases in size, owing to its particles being separated farther from each other: by continuing this influence, after a certain time the cohesion of the mass is so reduced that it melts, or becomes liquid, and, under the force of a still higher temperature, this molten metal may be dissipated in vapour. It would appear as if, under the agency of the heat applied to a body, its atoms expanded, until at last, owing to the tenuity of the outer layer or envelope of each atom, they were enabled to move freely over each other, or to interpenetrate without difficulty. That heat does really occasion a considerable disturbance in the corpuscular arrangement of bodies, may be proved by a very interesting experiment. A bar of heated metal is placed to cool, with one end supported upon a wedge or a ring of a different metal the other resting on the ground. In cooling, a distinct musical sound is given out, owing to the vibratory action set up among the particles of matter moving as the temperature declines.[68] Heat is diffused through all bodies in nature, and, as we shall presently see, may be developed in many different ways. We may, therefore, infer, that in converting a sphere of ice into water, and that again into steam, we have done nothing more than interpenetrate the mass with a larger quantity of heat, by which its atoms are more widely separated, and that thus its molecules become free to move about each other. Hence, from a solid state, the water becomes fluid; and then, if the expansive force is continued, an invisible vapour. If these limits are passed by the powers of any greatly increased thermic action, the natural consequence, it must be seen, will be the separation of the atoms from each other, to such an extent that the molecule is destroyed, and chemical decomposition takes place. By the agency of the electricity of the voltaic battery, we are enabled to produce the most intense heat with which we are acquainted, and by a peculiarly ingenious arrangement Mr. Grove has succeeded in resolving water by the mere action of heat into its constituent elements--oxygen and hydrogen gases. That this decomposition is not due to the voltaic current, but to the heat produced by it, was subsequently proved by employing platina heated by the oxy-hydrogen flame.[69]