HELIUM. 339 



HELIUM, ITS IDENTIFICATION AND PROPERTIES. 



By Pkof. C. a. young. 



THE famous " Dg/' so called because it is very near the D lines 

 of sodium, is a bright yellow line in the spectrum of the 

 solar chromosphere, in which it is more conspicuous than any- 

 thing except the C and F lines of hydrogen. Unlike them, how- 

 ever, it has no corresponding dark line in the ordinary solar spec- 

 trum, a rather perplexing fact which has caused much discussion, 

 and has not even yet found an explanation in which all authori- 

 ties agree. 



It was discovered in 1868, when the spectroscope was for the 

 first time directed upon a solar eclipse. Most of the observers 

 supposed it to be the sodium line, but Janssen noted its non-coin- 

 cidence ; and very soon, when Lockyer and Frankland took up 

 the study of the chromosphere spectrum, they found that the line 

 could not be ascribed to hydrogen or to any other known terres- 

 trial element. As a matter of convenient reference Frankland 

 proposed for the unknown substance the provisional name of 

 " helium " (from the Greek " helios," the sun), and this ultimately, 

 though rather slowly, gained universal acceptance. 



Within a year, two other lines (A. 7,005 and A. 4,472) were dis- 

 covered in the chromosphere spectrum by Rayet and Respighi 

 respectively, which like D, are always present in the prominences, 

 but have no corresponding dark lines. It was of course early 

 suggested, but without proof, that these lines might also be due 

 to helium. Since then some eight or ten other lines have been 

 found, frequently, but not always, presenting themselves in the 

 chromosphere spectrum, and, like the first three, also without 

 dark analogues. Moreover, still more recently, D3 and its con- 

 geners have been detected in the stellar spectra darl<. in the 

 spectra of the " Orion stars,"' bright in the spectra of certain 

 variables and of the so-called Wolf-Rayet stars ; and both bright 

 and dark in ^ Lyrse and the " new star " of Auriga which 

 appeared in 1892. 



Naturally there has been much earnest searching after the 

 hypothetical element, but until very recently wholly without suc- 

 cess; though it should be mentioned that in 1881, Palmieri, the 

 director of the earthquake observatory upon Vesuvius, announced 

 that he had found Dg in the spectrum of one of the lava minerals 

 with which he was dealing. But he did not follow up the an- 

 nouncement with any evidence, nor has it ever received any con- 

 firmation, and from what we now know as to the conditions neces- 

 sary to bring out the helium spectrum, there is every reason to 

 suppose that he was mistaken. 



