340 POPULAR SCIENCE MONTHLY. 



The matter remained a mystery until April, 1895, when Dr. 

 Ramsay, who was Lord Rayleigh's chemical collaborator in the 

 discovery of argon, in examining the gas liberated by heating a 

 specimen of Norwegian cleveite, found in its spectrum the D3 line, 

 conspicuous and indubitable. The mineral was obtained from 

 Dr. Hillebrand, one of our American chemists, who had previously 

 studied it, and ascertained that it could be made to give off a gas 

 which he identified with nitrogen^ It really was nitrogen in part, 

 but Ramsay suspected that he should also find argon, as he did 

 and helium besides, which was unexpected. 



Cleveite is a species of uraninite or pitchblende, and it soon 

 appeared that helium could be obtained from nearly all the ura- 

 nium minerals, and from many others ; from many, mingled with 

 argon ; from others, nearly pure. In fact, it turns out to be 

 very widely distributed, though only in extremely small quan- 

 tities, and generally " occluded," or else in combination sel- 

 dom, if ever, free. It has been detected in meteoric iron, in the 

 waters of certain mineral springs in the Black Forest and Pyr- 

 enees, and Kayser even reports traces of it in the atmosphere 

 at Bonn. 



It is generally obtained by heating the substance that contains 

 it in a close vessel connected with an air pump of some kind by 

 which the liberated gases are drawn off and collected. They are 

 then laboriously treated to remove as far as possible all the for- 

 eign elements (nitrogen, etc.), since the presence of no more than 

 five or ten per cent of other gases prevents the new elements from 

 giving any spectroscopic evidence of their presence ; they are too 

 shy and modest to obtrude themselves. In many cases, as has 

 been said, argon and helium come off together, and certain lines 

 in their spectrum are nearly coincident, so that for a time there 

 was supposed to be some close bond of connection between them. 

 The latest observations, however, make it certain that this is not 

 so : as Mr, Lockyer puts it, " argon is of the earth, earthy, but 

 helium is distinctly celestial." 



Its spectrum has been thoroughly studied by Crookes, Lockyer, 

 and Runge, who agree as to all its leading characteristics. 



Runge, whose work is the most complete and authoritative, 

 finds that its lines have a remarkably regular arrangement, fall- 

 ing into two distinct " sets," each set consisting of a principal 

 series and two subordinate ones, the lines in each series corre- 

 sponding very accurately to a formula quite similar to that dis- 

 covered by Balmer as governing the hydrogen spectrum. 



In the whole spectrum he finds (by photography mainly, 

 though two of the most important were detected by the bolom- 

 eter) sixty-seven lines, twenty of which only are in the visible 

 part of the spectrum. Of the sixty-seven, twenty-nine belong to 



