250 PHENOMENA DEPENDENT ON MOLECULAR PATHS 93 



diffusion of the second gas into the first depends. For 

 since no inequality in the total pressure is brought about 

 by diffusion in an experiment wherein the initial pressures of 

 the two diffusing gases are equal to each other, the strengths 

 of the two flows which occur in opposite directions, but 

 otherwise under the same circumstances, must be equal, 

 and, therefore, also the two coefficients must be equal. 



Loschmidt's observations showed that the value of 

 this coefficient is inversely proportional to the total pressure 

 of the gases, so that, as is almost obvious, the mixing occurs 

 the more rapidly the more rarefied the gases. Loschmidt 

 found further that the coefficient alters with the temperature 

 in accordance with the law, that it increases nearly propor- 

 tionally to the square of the absolute temperature. Finally 

 it appeared that a regular relation must exist with the mole- 

 cular weights, which, however, did not succeed in disclosing 

 itself with full clearness. 



We have to develop the value of this coefficient from 

 the conceptions of the kinetic theory. 



94. General Theory of Diffusion 



The slowness with which two diffusing gases mix 

 together is to be explained on the kinetic hypothesis in a way 

 which is so fully analogous to the conceptions underlying 

 the theory of friction that we may speak of diffusion as a 

 kind of reverse friction of the two gases on each other. 



The cause of the phenomenon, that the forward flow of 

 one layer is transmitted only very slowly by friction to a 

 distant layer, lies only in the shortness of the molecular 

 path. With all their swiftness of motion, the molecules 

 transmit a part of their own momentum only to those 

 particles that are quite close to them and with which they 

 collide after traversing a very short path. A transference of 

 momentum to a greater distance takes place, therefore, only 

 by the interaction of very many particles in their to-and-fro 

 motions ; it is, consequently, carried on by no means in a 

 straight direction, and experiences, therefore, a considerable 

 retardation, which appears to us as a consequence of friction. 



