Separation of the Least Volatile Gases of Atmospheric Air, &c. 393 



temperatures depends on keeping the temperature of B as low as 

 possible, as will be seen from the following consideration : — 



The pressure p, of a gas G, above the same material in the liquid 

 state, at temperature T, is given (approximately) by the formula 



logp = A - — , 



where A and B are constants for the same material. For some other 

 gas G' the formula will be 



logp! = A 1 -^ i 

 and l og ^ = A-A 1 + ?l^. 



Pi -L 



Now for argon, krypton, and xenon respectively the values of A are 

 6-782, 6-972, and 6-963, and those of B are 339, 496-3, and 669-2 ; so 

 that for these substances and many others A - A\ is always a small 

 B — B 



quantity, while — is considerable and increases as T diminishes. 



Hence the ratio of p to jh increases rapidly as T diminishes, and by 

 evaporating the gases always from the solid state and keeping the solid 

 at as low a temperature as possible, the gas first removable at the 

 lowest pressure consists in by far the greatest part of that which has 

 the lowest boiling point, which in this case is nitrogen, and is suc- 

 ceeded, with comparative abruptness, by the gas which has the next 

 higher boiling point. By this method the nitrogen and oxygen are 

 removed without the necessity of sparking or absorption. The 

 change from one gas to another is easily detected by examining the 

 spectrum in the sparking tube, and the reservoirs into which the gases 

 are pumped can be changed when the spectrum changes, and the frac- 

 tions separately stored. Or, if several sparking tubes are interposed 

 in such a way as to form parallel communications between the tubes b 

 and e, any one of them can be sealed off at any desired stage of the 

 fractionation. 



The variation of the spectra of both xenon and krypton with varia- 

 tion in the character of the electric discharge is very striking, and has 

 already been the subject of remark, in the case of krypton, by Runge, 

 who has compared krypton with argon in its sensitiveness to changes 

 in the electric discharge. Runge distinguishes krypton rays which are 

 visible without a jar and those which are only visible with a jar dis- 

 charge. The difference in the intensity of certain rays, according as 

 the discharge is continuous or oscillatory, is no doubt very marked, 

 but, with rare exceptions, we have found that the rays which are 

 intensified by the oscillatory discharge can be seen with a continuous 



E 2 



