Page:Popular Science Monthly Volume 48.djvu/373

Rh the first "set" and thirty-eight to the second. Of the twenty "visual" lines, thirteen have been observed in the spectrum of the chromosphere; the missing lines all belong to the second subordinate series of the first "set," and are so faint in the artificial spectrum of the gas that their failure to be found in the chromosphere needs no explanation.

The fact that the lines thus divide into two mathematically independent "sets" has led Runge to believe that the helium obtained from the minerals is really a mixture of two distinct gases, and he has found it possible to partially separate the two by a process of diffusion. The true helium, the element that gives D3 and the other lines that are always present in the chromosphere spectrum, he considers to be the denser of the two; the spectrum of the other contains most of the lines that appear only occasionally in prominences. The lighter component has as yet received no name. Lockyer calls it simply X.

The lines of the series to which D3 belongs are all double, having a very faint companion on the lower (i. e., red-ward) side, extremely close to the principal line. When Runge announced this discovery early in June it at first produced something like consternation among spectroscopists, for at that time there still remained more or less doubt as to the validity of Ramsay's identification, and the solar D3 had never been observed to have such a companion. Very soon, however, Hale, Huggins, Lockyer, Reed of Princeton, and other observers who had sufficiently powerful instruments, detected the little attendant of D3 in the spectrum of prominences, so that the momentary distrust was replaced by absolute confidence.

As to the physical and chemical properties of the new gas, our knowledge is still limited and our conclusions are embarrassed by the uncertainty whether we are dealing with a single element or a mixture—whether Dr. Ramsay has introduced to the world one infant or a pair of twins.

The gas liberated from clèveite, and purified as far as possible, shows a density just a little more than double that of hydrogen, and is therefore much lighter than any other known gas except hydrogen itself. If it is a mixture, the lighter gas must have a density less than two, and may even prove to be lighter than hydrogen; while the true D3 helium may have a density anywhere between two and four, depending on the proportions of the mixture and the density of the lighter compound. In any case both the true helium and X are lighter than anything else but hydrogen.

It would be very fine, we may remark in passing, if the lighter component could have been identified with "coronium," but this seems impossible since the characteristic 1,474 line (λ 5,316) does