272 PHENOMENA DEPENDENT ON MOLECULAR PATHS 101 



action of similar molecules, while a denotes the distance 

 apart of dissimilar molecules when they collide and, there- 

 fore, the radius of this sphere for the action of dissimilar 

 particles on each other. If, now, as we saw in our discus- 

 sion of viscosity, the diminution of the radius s of the sphere 

 of action with rise of temperature is only apparent, and is 

 to be referred to forces of cohesion, as with Sutherland's 

 explanation, we shall have to assume, in the same 

 way, that the magnitude a also appears to diminish with 

 rise of temperature only because the forces of cohesion, 

 or, more accurately, the attractive forces exerted by dis- 

 similar molecules on each other, heighten the probability of an 

 encounter, and, indeed, increase it the more strongly the less 

 the speed fl of the molecular motion or the lower the tem- 

 perature. Accordingly the law of dependence of the co- 

 efficient of diffusion on the temperature must be represented 

 by a formula which must be formed quite similarly to that 

 found for viscosity ( 85), viz. 



77 = 



we shall therefore have to put 

 D = D (l + o3)*(l 



where a represents the coefficient of expansion, and 6 the 

 absolute temperature 



= 9. + a- 1 = 9 + 272-5, 



while D stands for the value of D at C., and G' is 

 a constant which represents the measure of the cohe- 

 sion between dissimilar particles, as does C for similar 

 particles. 



We may omit a detailed comparison of this theoretical 

 formula with the observations; for, since Sutherland's 

 formula has been proved for viscosity, the agreement of the 

 values of the exponent n now shows that the formula for the 

 diffusion can represent the results of experiment in the same 

 excellent way. It is therefore sufficient to give the values 



