138 SIR WILLIAM RAMSAY 



This suggests that atmospheric argon contains, besides argon, 

 some other gas which has as yet not been separated and which 

 may possibly account for the anomalous position of argon in its 

 numerical relations with other elements. 



Not having a spectroscope with which accurate measurements 

 could be made, I sent a tube of the gas to Mr. Crookes, who has 

 identified the yellow line with that of the solar element to which 

 the name ' helium ' has been given. He has kindly undertaken 

 to make an exhaustive study of its spectrum. 



I have obtained a considerable quantity of this mixture and 

 hope soon to be able to report concerning its properties. A 

 determination of its density promises to be of great interest." 



It is scarcely necessary to remark that at that time 

 no one expected to make a close acquaintance with 

 helium as a terrestrial element. The line D 3 charac- 

 teristic of the element supposed by Lockyer to exist 

 in the sun had been measured many years previously by 

 Angstrom and by Cornu and their estimates were now 

 confirmed by the work of Crookes, who gave the wave- 

 length as 587'45. 



, The mineral cleveite is a variety of uraninite, and 

 minerals containing the element uranium were found 

 very generally to yield more or less helium together 

 with hydrogen, nitrogen and other gases. Helium was 

 found to be like argon, chemically inert. Its density 

 is a little less than twice that of. hydrogen, namely T99 

 and it is composed of monatomic molecules, like those 

 of argon. Its molecular weight is therefore approxi- 

 mately 4 and the symbol is He. It may be added here 

 that all attempts to liquefy helium remained for many 



