438 



the water vapour is condensed. Tlie gas mixture to be separated is 

 admitted through the tube G. A part of this mixture diffuses against 

 the current tlirongh the fioles in D ; this part can be pumped off with 



part of the water vapour through the tube 

 H. The temperature of the water in the 

 cooling jacket must be regulated in sucii a 

 way, that the sum of the partial pressure 

 of the water vapour and the pressure of 

 the gas mixture in the vessel V is exactly 

 so much smaller than the pressuie of the 

 water vapour admitted through the tube, 

 that the required current velocity is obtained. 

 The appliances used to attain this regulation, 

 will be discussed later. The method described 

 has so far beeFi chiefly used to sepaiate 

 helium-neon mixtures, and has proved very 

 satisfactory. Even, when the process of 

 diffusion was executed only once, from such 

 a mixture containing 30 '/o helium, helium 

 could be obtained, the purity of^ which was 

 so great, that in a Geissler-tube at a pressure 

 of 1 m.m. the neon-lines were not visible 

 with an ordinary spectroscope. Considering 

 the exceedingly great spectral sensitiveness 

 of Helium with regard to very small quan- 

 , titles of Neon, this shows already a very 

 great degree. of purity. 



Though the unmixing of the gas mixture 

 by diffusion against the gas current was 

 actually as great as was to be expected according to theory, the 

 quantities obtained remained below expectation. This may be ex- 

 plained by considering, that in the method described only part 

 of the cross section of the vapour current is used, because the gas 

 must diffuse from the outside into the jets that issue separately 

 from each hole. In order to deal with greater quantities another 

 apparatus appeared to be »nore suitable, working according to the 

 second example discussed above. This second case is in so far 

 much more easily I'ealized, as it is not necessary here (o keep the 

 current velocity accurately constant. It is immediately seen that with 

 a current as represented in fig. 1, also unmixing of a mixture is 

 to be expected, when this mixture is introduced at a point in 

 the axis of symmetry of the current. The principal part of the 



Fig. 3. 



