262 PHENOMENA DEPENDENT ON MOLECULAR PATHS 98 



D therefore assumes a different value at every different place 

 in the mixture that is being formed by the diffusion ; its 

 values consequently alter with the rate of fall of pressure of 

 each of the two gases, so that for the same reason the 

 coefficient is variable with the time during the whole period 

 of the experiment. So the observed values of the coefficient 

 D will not be able to exhibit any perfect agreement together 

 if the observations are made for unequal values of the ratio 

 of mixture N { : JV 2 . 



The theoretical calculation of an experiment on diffu- 

 sion therefore becomes very complicated, and so excessively 

 laborious that we easily comprehend why the foregoing 

 formulae, which were deduced in the first edition of this 

 book, have scarcely at all been used up to now. People 

 have for the most part preferred to employ less exact but 

 more convenient formulae, which give a constant value for 

 the coefficient of diffusion. 



99. Another Theory of Diffusion 



The theory leads to a constant value of the coefficient 

 of diffusion if a somewhat different fundamental assumption 

 is made as to the cause of the slowness with which the 

 mixture of the two gases proceeds. There can be no doubt 

 but that this cause must be sought in the mutual encounters 

 of the particles, which prevent a forward path in a straight 

 line. But the question may be raised whether the two kinds 

 of molecules take part in these processes in the same way 

 and in equal measure. 



It does not seem improbable to assert that the encounters 

 between molecules of the same kind have little influence on 

 the velocity of the current with which each of the two gases 

 flows against the other ; that just as often as a molecule 

 loses velocity by an encounter with another of the same 

 kind, it happens that it gains velocity in the direction of the 

 flow. Quite otherwise, on the contrary, is the case with 

 encounters between particles of different kinds. Since both 

 gases are streaming in different directions, the final result 

 of the encounters between particles of different kinds must 



