[ 83 ) 



Vapour pressure of the less volatile elcuieiit at the temperature of 

 cooling. The separation will be much more perfect when we can 

 also avail ourselves of compression, as it is the case, for instance, 

 when the gas which we want to purify, at the lo^vest temperature 

 to which we can cool, is still above its critical temperature. 



If we do not take the pressure too high we may assume roughly 

 that the degree of purity which we can reach with long continued 

 cooling, is at the same temperature directly proi)ortional to the 

 pressure to which we compress. In cases where the gas flows through 

 a cooled tube, other factors come into consideration, but even then 

 compression offers a great advantage. 



I have availed myself of this operation for a last and thorough 

 purification of the electrolytic hydrogen (prepared as described in 

 Comm. N". 27, May '96), which is used for piezometers and thermo- 

 meters, when it appeared that notwithstanding it was led through 

 drying tubes with phosphorus pentoxide, traces of water still occurred 

 in the gas. This purification was effected by cooling hydrogen under 

 strong pressure in liquid air. 



A similar method may be recommended to free, for instance, helium 

 from admixtures of neon and hydrogen. The degree of purity of 

 the helium can be raised considerably by causing the bath (liquid 

 hydrogen) to evaporate in vacuo; for this purpose an apparatus is 

 being constructed ^). 



§ 2. Pure hydrogen for thermometers and piezometers. 



Several improvements have been made to the apparatus for the 

 preparation of pure hydrogen (described in Comm. N". 27). Some 

 of them are described in Comm. N". 60, Sept. '00. Later the [dale 

 ƒ of fig. 6, PI. II, Comm. N". 27, was riveted to a platinum wire 

 (instead of being soldered to the copper wii-e e) and melted in a 

 glass tube which is bent down under the mercuiy on the bottom of 

 the apparatus and is itself filled with mercury. Further the cock d 

 was sealed to the bell-jar c, and the sealing place k is kept under 

 mercury to be cooled by it; finallj^ the shutting of the apparatus was 

 made easier as the India rubber stoppers in the cover were replaced 

 by cone-shaped ones which are pressed on to it by means of a 

 small plate and tightening screws and as six tightening rods t 

 instead of three as in the above mentioned figui'e have been made. 



1) After this had been wiillen and published in the Dutch Proceedings of the 

 Academy I found that Dewar in his Bakerian Lecture, Proc. Roy. Soc. Vol. G8, 

 1901, recommended the method of adding compression to cooling for purifying 

 helium. 



