HELIUM. 341 



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 sub- 

 ordinate series of the first " set," and are so faint in the artificial 

 spectrum of the gas that their failure to be found in the chromo- 

 sphere needs no explanation. 



The fact that the lines thus divide into two mathematically 

 independent " sets " has led Runge to believe that the helium ob- 

 tained 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 occasion- 

 ally 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, hav- 

 ing a very faint companion on the lower (i. e., red- ward) side, ex- 

 tremely close to the principal line. When Runge announced this 

 discovery early in June it at first produced something like con- 

 sternation among spectroscopists, for at that time there still re- 

 mained more or less doubt as to the validity of Ramsay's identifi- 

 cation, 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 ab- 

 solute 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 cleveite, 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 any- 

 where 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 (A 5,316) does 



