INERT CONSTITUENTS OF THE ATMOSPHERE. 593 



and one in the green part of the spectrum. The third gas, named 

 xenon, or 'the stranger/ gives out a greenish-blue light, and is remark- 

 able for a very complex spectrum, in which blue lines are conspicuous. 



Although neon vras first obtained by the fractional distillation of 

 argon, it was afterwards found convenient to prepare it direct from 

 air. The torpedo-compressor, which is used for compressing the air 

 before it enters Dr. Hampson's liquefier, was made to take in the air 

 v/hich had escaped liquefaction in the liquefier; the denser portions 

 were thus liquefied, and the lighter portions were liquefied by compress- 

 ing them into a vessel cooled by the denser fractions, boiling under 

 reduced pressure, and consequently at a specially low temperature. 

 This liquefied portion was again fractionated, and yielded neon; and 

 it was not long before we discovered that helium was also present in 

 the mixture. The presence of helium in atmospheric air had previously 

 been noted by Professor Kayser of Bonn, and by Professor Friedlander 

 of Berlin, on submitting the spectrum of argon to a searching ex- 

 amination. 



The purification of this mixture of neon and helium from argon, 

 although a lengthy process, was not attended by any special difficulty. 

 It was accomplished by repeated distillation, the lighter portions being 

 always collected separately from the heavier portions, and again dis- 

 tilled by themselves. But after this separation had been accomplished, 

 we found that we were unable by means of liquid air to liquefy the 

 mixture, or indeed any portion of it. We effected a partial separation 

 by diffusion; but it is not possible to separate by this method two 

 gases of which the quantity is limited. Another attempt was made 

 by dissolving the gases in liquid oxygen, on the supposition that neon 

 might prove more soluble than helium; but without satisfactory re- 

 sults. It was evident that a lower temperature than that possible by 

 help of liquid air was necessary. 



Professor Dewar had by that time succeeded in producing liquid 

 hydrogen in quantity, and had indicated the principle, which is identical 

 with that of Dr. Hampson's air-liquefier, although he has not published 

 any detailed account of his apparatus. Dr. Travers undertook to 

 investigate the subject; and after four unsuccessful trials, he made 

 a liquefier, with the help of Mr. Holding, the laboratory mechanician, 

 by means of which a hundred cubic centimeters of liquid hydrogen 

 could be easily and cheaply produced. There was then no difficulty 

 in effecting the separation of neon from helium; for, while neon is 

 practically non-volatile, when cooled by liquid hydrogen, remaining in 

 the state of solid or liquid, even that enormously low temperature is not 

 sufficient to convert helium into a liquid. Hence the gaseous helium 

 could be pumped away from the non-gaseous neon, and the latter was 

 obtained in a pure state. 



VOL. LIX. — 41 



