34 6 HISTORY OF CHEMISTRY. [LECTURE XVI. 



rendered an extremely difficult one, since it falls near that of 

 potassium and yet is beyond it. 



Ramsay took up the problem from a very general point of 

 view. It appeared to him highly probable that argon was a 

 member of a whole group of elements, of which group he 

 hoped to find additional members associated with nitrogen. 

 It was thus that he came to investigate, amongst other things, 

 the gases evolved from cleveite by heating with sulphuric acid, 

 which Hillebrandt had considered to be nitrogen, 70 and this 

 led him to the discovery of helium. The brightest line in the 

 spectrum of this gas, D 3 (Dj and D. 2 are the sodium lines), had 

 been observed a long time previously by Lockyer in the 

 spectrum of the sun's photosphere. 71 Helium, whose atomic 

 weight 4 was deduced from the density of the gas and from 

 the rate of propagation of sound in it, was an analogue of 

 argon in every respect ; and it was thus clear to Ramsay that 

 there must be another element which, with atomic weight 

 about 20, should be placed before sodium, in the same way 

 that helium comes before lithium, and argon probably before 

 potassium, although the atomic weight of argon has been 

 found, in the meantime, somewhat higher than that of potas- 

 sium. 72 A similar thing applies to tellurium, the atomic 

 weight of which, according to the most recent determinations, 

 is greater than that of iodine. 73 



Ramsay now represents the further development of the 

 subject 74 as if the investigation, carried out with his utmost 

 energy and effort, had remained unproductive, and as if an 

 accident only had led him on to his further discoveries. There 

 is in reality, however, no such accident in question, for the 

 investigation of the residue from the evaporation of liquid air 

 was only a link in the chain which, although perhaps unknown 

 to himself, represented the course of his ideas. In this way 

 he discovered crypton, the molecular weight of which was 



70 Bull. U.S. Geological Survey, 78, 43- 71 Nature, 53, 319. 



72 Berichte. 31, 3111. 7:$ Brauner. Journ. Chem. Soc. 67, 549. 74 Berichte, 



