292 



J. Deivar — Separation of Gases from Air. 



Now the vacuum just reached the beginning of the phosphor- 

 escent stage. With hydrogen, either a pressure of gas less 

 than that of the atmosphere had to be used at starting or a 

 larger amount of charcoal employed in order to get a vacuum 

 well up in the strise stage. If, however, the liquid air was 

 cooled to —210° C. by exhaustion, the tube just reached the 

 beginning of phosphorescence round the cathodes. 



With helium there was a very slight absorption, but neon 

 did show T something more appreciably. Spectroscopic observa- 

 tions made during the condensation of the gas in the charcoal 

 showed the gradual disappearance of the characteristic spec- 

 trum of oxygen, nitrogen and air, as the high vacuum was 

 reached and the discharge passed with great difficulty. In 



c 



<^ I i 



J K 



tubes of this kind filled at atmospheric pressure I could always 

 see the F line of hydrogen and the neon yellow ; but the 

 helium was not seen with any definiteness. As the amount of 

 neon in the air cannot well exceed l/50,000th the spectroscopic 

 test is very delicate. 



In order to bring in the helium lines it was necessary to 

 concentrate the volume of air in the space of the sparking tube 

 six or seven times. This was done by the use of an arrange- 

 ment shown in fig. 2. AB is the sparking tube with its small 

 charcoal bulb C attached, capable of being sealed off when 

 required at G ; and D and E are larger charcoal absorbers 

 placed in vacuum tubes containing liquid air ; the whole being 

 attached to a graduated gas-holder containing air. A series of 

 glass stop-cocks are attached at the points H, I, J and K in 

 order to facilitate manipulation. In determining the volume 

 of air required to bring in the helium lines, only one charcoal 

 absorber containing about 15 grams of material was used. On 

 allowing 200 c.c. of air from the gas holder to be sucked into 

 the charcoal (which had been previously exhausted along with 

 the sparking tube) on opening the stop-cock H any residuary 



