formed in equal times, and to this extent the molecule behaves as an elastic body. The pulses which it imparts to the ether just possess the properties requisite for the production of light.
It is, however, necessary to suppose that the vibrations which the molecule communicates to the ether are not solely of one type. Were this the case only one set of waves would be propagated, and there would only be a single line in the spectrum. In the spectrum of hydrogen, however, there are, as we have seen, quite a number of lines. These seem to resemble harmonics of some fundamental note. It would, therefore, appear as if the molecule of hydrogen, in addition to its fundamental vibration in the note which properly belongs to it, possessed a number of subsidiary vibrations which might properly be regarded as harmonics. The molecule is thus acting in much the same way as a musical instrument, which, in addition to the primary note to which it mainly responds, disperses at the same time and in consequence of the same impulse a number of fainter notes which are harmonics of the fundamental one.
It is quite evident that the molecule of hydrogen must be of a much simpler character than the molecule of many other elements. For though the spectrum of hydrogen contains a large number of lines, most of them, if not indeed all, belong to a group of associated vibrations. In the case of other elementary bodies the complexity of the spectrum is such as to make us think that the vibrations of the molecule must be of a very complicated character. Thus in the case of iron, we find that when this element is brought to the gaseous state by heat, the light which it emits has a spectrum containing some thousands of lines.
