Another most interesting discovery is that of helium, which was made by Professor Ramsay, in 1895, while examining the mineral cleveite for argon, when, besides the spectrum of argon, he observed another bright line not belonging to that spectrum, which Mr. Crookes recognized as identical with the line D3 which Professor Lockyer had observed in 1868 in the spectrum of the solar chromosphere, and which he attributed to an element not yet known on the earth—helium. The same line was afterward found in the spectra of other fixed stars, and the conclusion was drawn that helium exists in large quantities outside of the earth. On our planet it seems, however, to be very rare, and may even be ranked among the rarest elements. Yet it has been almost discovered several times. Palmieri observed the line of helium in his researches on the lava of Vesuvius, but did not push the matter further; and Hillebrand in 1891 obtained in the spectrum of the gas formed by uranite lines which were presumptively those of helium. Since its discovery, helium has been found in a considerable number of minerals, generally associated with uranium, yttrium, and thorium; in mineral waters and, in extremely small quantities, in atmospheric air. Next to hydrogen, it is the lightest of the gases, and from this peculiarity Stoney draws an explanation of the fact that these two elements exist only in very small quantities in a free state on the earth, while they are diffused in enormous masses through the universe. The relatively small force of the earth's gravitation does not furnish an adequate counterpoise to the velocity of their molecules, and they escape from the atmosphere of the earth, unless they are restrained by chemical combination. They then collect around the great centers of attraction constituted by the stars, in the atmosphere of which they exist in large quantities.
The study of the spectrum of helium is extremely important, because it gives interesting data concerning the nature of distant celestial bodies. It also, as the labors of Runge and Paschen have shown, suggests doubts concerning the elementary character of the new substance. Whatever it may be, if we have to suppose that helium is composed of two gases (Mr. Lockyer has proposed the name of asterium for the second), one of the two gases probably has a boiling point very near the absolute zero, and in any case below -264° C; for the master in liquefaction of gases, M. K. Olszewsky, has not up to this time succeeded in provoking a change of state of helium, and he proposes to use this gas for filling gas thermometers for measuring extremely low temperatures. Helium has shown itself thus far as refractory as argon to all chemical combination, and so great an uncertainty reigns over the position to be attributed
