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 It may no doubt be true that groups of these correspond to harmonics of a smaller number of fundamental notes, but even with this admission the vibrations of the molecule of iron must necessarily be of a highly elaborate description.

The effect of heating a gas is to make its molecules move more rapidly. When this happens the collisions between the molecules take place with greater vehemence, and the internal agitations of the molecules arising from the shocks of their collision are all the more intense. Hence, when the gas becomes hot enough, the molecules vibrate sufficiently to produce undulations in the ether strong enough for the perception of light.

Much of what has been said with regard to light may be repeated with regard to heat. We know that radiant heat consists of ethereal undulations of the same character as the waves of light. Hence, we see that the heat or the light radiated from a glowing gas is mainly provided at the expense of the energy possessed by the molecules in virtue of their internal oscillations.

One of the most instructive applications of these principles is to afford an explanation of the means by which the sun sustains its heat, of which we have already spoken in a previous chapter. As the great luminary is a mass of glowing gas or vapour, it is of much interest to examine how far the doctrine of the molecular theory of gases can answer the great question of solar physics. It has of course been long known that the sun retained its power of radiating heat for so many ages in virtue of its contraction. Helmholtz had shown that the amount of potential energy due to gravitation in a mass of matter equal to that of the sun, and expanded over a volume a