184 Dr. S. Chapman on the 



For equal values of log p/p' and log T'/T, pressure diffusion 

 is more powerful than thermal diffusion in the ratio 



k P /k T = ^ '"'t? (9-15-8-25 \{K 2 ). 



Tn the case of neon and metaneon this varies from 2*5 

 when the gases are equally mixed (X 1 = \ 2 = ^) to about 

 3*2 when there is a large excess of either gas. Thus, if it 

 were equally easy to obtain a given ratio for either p or T, 

 pressure diffusion would afford much the more advantageous 

 method of varying the relative concentration of the two 

 g;ases. It seems more convenient, however, to maintain 

 large differences of temperature than of pressure, and it 

 may therefore be preferable lo use the feebler type of 

 diffusion on this account. The operation can, of course, be 

 repeated any number of times. 



§ 3. To take a numerical example, suppose that it is 

 desired to separate a mixture of equal proportions of neon 

 and metaneon. Then, writing wij = 22, m 2 — 20, X x =\ 2 = i> we 

 find that & T = 0*0095. Suppose that the mixture is placed 

 in a vessel consisting of two bulbs joined by a tube, and one 

 bulb is maintained at 80° absolute by liquid air, while the 

 other is heated to 800° absolute (or 527° C). When the 

 steady state has been attained the difference of relative 

 -concentration between the two bulbs is given by the equation 



\ x -V=-(x 2 _\ 2 ') = 0-0095 log. 800/80, 

 = 0-0095 log, 10, 

 = 0-022, 



or 2*2 per cent. Thus the cold bulb would contain 48*9 per 

 cent, metaneon to 51*1 per cent, of neon, and vice versa in 

 the hot bulb. By drawing off the contents of each bulb 

 separately, and repeating the process with each portion of 

 gas, the difference of relative concentrations can be much 

 increased. But as the proportions of the two gases become 

 more unequal, the separation effected at each operation 

 slowly decreases. For instance, when the proportions are 

 as 3:1, the variation at each such operation falls to 1*8 

 per cent., in place of 2'2 per cent. ; while if X] :\ 2 is 10 : 1, 

 the value is 1'2 per cent. This assumes that the molecules 

 behave like elastic spheres : if they behave like point centres 

 of force varying as the inverse nth power of the distance, 

 the separation is rather less ; e. g., if w = 9, it is just over 

 half the above quantities. 



