530 Dr. F. A. Lindemann and Dr. F. W. Aston on the 



started again, and when another 100 c.c. are collected the 

 whole will be diffused again. This second operation should 

 give a theoretical enrichment of 2, so that unless the mixing 

 is very bad a definite increase of density might be obtained. 



A very much simpler and better type of continuous 

 diffusion apparatus would be one on the rotating principle 

 used in the Gaede mercury pump, the difficulty being the 

 manufacture of the rotating part of pipeclay or unglazed 

 porcelain with a suitable device for withholding and collect- 

 ing the small residual volume of less diffusible gas. Should 

 diffusion methods prove sufficiently hopeful, an effort will be 

 made to construct such an apparatus. 



It will be seen from Rayleigh's equation above that there 

 is not the least hope of getting approximately complete 

 separation for a reasonable quantity of gas in a reasonable 

 time by this diffusion method even under theoretically 

 perfect conditions of mixing. 



Another method that has been suggested consists in allow- 

 ing the gas to diffuse through a column of heavier gas or 

 liquid. Its practical difficulties are considerable, and its 

 advantages over the other method by no means obvious. 



Density. 



There are two direct methods of separating isotopes by the 

 effect of their different masses. The first of these, gravi- 

 tational separation in the stratosphere, applies only to neon 

 if this be really a mixture of two isotopes. 



If M be the atomic weight, g the gravitational constant, 

 p the pressure, and p the density, then if no mixing takes 

 place dp——gpdh, h being the height. In the isothermal 

 layer convection is small. If it is small compared with 

 diffusion the gases will separate to a certain extent. Since 

 T is constant 



UTp . dp M</ 7 j 



p=m~ and 7 = Rt ' 



whence p=p e RL » 



p being the density at the height 7i at which mixing by 

 convection ceases, about 10 kilometres, and Ah the height 

 above this level. If two isotopes are present in the ratio 1 

 to K , so that the density of one is p and of the other K ^ Q 



