93 DIFFUSION OF GASES 249 



of the idea of a resistance to diffusion which each gas 

 experiences from the particles of the other gas which meet 

 it. This resistance is taken to be proportional to the 

 densities of both gases and to the difference of their 

 speeds of diffusion. The working out of this idea leads 

 to formulae which have a great likeness to the equations 

 which come into Fourier's theory of the conduction of 

 heat. 



This similarity is not only in respect of the mathe- 

 matical form, but is founded on the nature of the matter. 

 Just as heat spreads in a conducting body, so in diffusion a 

 gas spreads from one region to another. The speed with 

 which heat is transmitted is determined for each substance 

 by a constant which is termed the conductivity ; in like 

 manner the speed with which one gas penetrates into 

 another is determined by a magnitude which we might 

 call the diffusivity, but which is more usually termed the 

 coefficient, or constant, of diffusion. 



The meaning of these two constants is quite analogous. 

 We obtain the strength of the flow of heat by multiplying 

 by the conductivity the difference of the temperatures at two 

 places, which are distant from each other by unit length along 

 the line of flow, i.e. the so-called rate of fall of temperature. 

 We likewise obtain the intensity of the flow of diffusion if 

 we multiply by the coefficient of diffusion the difference of 

 the density of the diffusing gas at two places whose distance 

 apart is equal to unit length. But we can express the 

 meaning of this coefficient also in a somewhat different way 

 by replacing the density by the pressure which, by Boyle's 

 law, is proportional to it. We may then say that the 

 amount of partial pressure of one of the gases transmitted 

 by the diffusion is given by the difference in the values of 

 this partial pressure at two places which are separated by 

 unit length (or, as we may say more shortly, by the Tate of 

 fall of the partial pressure), multiplied by the coefficient of 

 diffusion. 



From this determination of the flow of diffusion it is easy 

 to see that the coefficient, which holds for the diffusion of 

 one gas into any other, must be equal to that upon which the 



