DIFFUSION OF GASES THROUGH PORES 



93 



given by the amount of the gas absorbed by the reagent 

 in a given time. The rates for different sizes of pore could 

 then be compared with each other and with the rate for the 

 uncovered tube. The results are reproduced in Table XXII. 



TABLE XXII 



Rate of Diffusion of Carbon Dioxide through Pores of Different 



Sizes 



From this it appears that the rate of diffusion is pro- 

 portional to the diameter (or radius, or circumference), 

 and not to the area. Now the diameter is proportional 

 not to the area but to its square root. Therefore if we 

 consider the case of two pores a and 6, of which b has an 

 area one half that of a, it follows that the diffusion through 

 b will be, not one half that through a, but seven tenths : 

 that is, the smaller the pore the greater is the diffusion per 

 unit area. This is demonstrated by the figures in column 3 

 of Table XXII. This applies to the diffusion of any 

 gas, to water vapour as well as to carbon dioxide, either 

 outwards or inwards through a pore. 



The reason of this relation may perhaps be appreciated 

 by thinking of the way in which water leaves an upturned 

 pipe under low pressure. It wells out over the edge of 

 the orifice, and the amount leaving the pipe will depend 

 rather on the length, or circumference, of that edge than 

 on the area of the orifice. In diffusion the gas wells over 

 the edge of the pore, though it diffuses upwards as well as 

 outwards. Browne and Escombe picture it passing through 



