able in some degree to learn certain properties of the vibrations with which it pulsates. Let us take, for instance, the element hydrogen and see what can be ascertained with respect to the molecules of that gas and their vibration. For the purpose of the experiment hydrogen in a state of extreme rarification is put into a tube, and a current of electricity from the induction coil is passed through it. The gas begins to glow with luminosity, and when the light is transmitted through the slit of the spectroscope lines characteristic of hydrogen are displayed. The visible lines are now known to be only a small part of the total spectrum of this element. For when the radiation from glowing hydrogen, either as obtained from a terrestrial source or from some of the stars, is photographed after passing through the prism, several lines are indicated that do not consist of light visible to the eye though visible to the peculiar sensibility of the photographic plate.
This system of spectral lines, so characteristic of hydrogen, must arise in some way from the molecules of which we know the gas to be constituted. We shall, therefore, consider how such effects are produced. A bright line, such as one of those of which the hydrogen spectrum is composed, arises from vibrations in the ether of one definite refrangibility. The effect of transmitting light through a prism is to sort out the different rays in accordance with their several refrangibilities. When, therefore, the spectroscope shows that light from incandescent hydrogen resolved into a number of bright lines, it thereby demonstrates that the radiation emitted from the glowing gas consists of just so many rays of the particular refrangibilities to which those lines correspond. But the re-
