95 DIFFUSION OF GASES 253 



unit area of a section from one side and pass through it, 

 would by 82 be 



i-ZVA and i-tfaflj, 



for the two sorts of molecules, whose mean speeds are fl l and 

 O 2 respectively. 



In consequence of the unequal distribution there come 

 fewer particles of the first kind from the side of the smaller 

 x and more in the opposite direction, while for the other 

 gas the reverse holds good. The particles of both groups 

 start from layers which are distant from the section with 

 abscissa x by magnitudes that are less than the possible 

 values of the free path, or are at most equal to them. The 

 average distance of these starting layers from the section 

 considered will be less than the mean free path, since "a 

 larger part of the particles not only reach this section but 

 pass through it. 



If we denote by A x the value of this mean distance of 

 the layers from which the particles of the first gas that 

 cross the section come and this value we shall determine 

 later those which cross in the positive direction start from 

 layers which, on an average, are distant by x \ from the 

 origin of the tube ; the density in these layers is not deter- 

 mined by N l but only by 



91 + n(x - AJ = JVj - wA, 



particles in unit volume. From this side, therefore, there 

 are only 



particles of the first kind which in unit time meet unit area 

 of the section ; there similarly cross this unit area in the 

 same time from the opposite side the number 



of the same kind, which start, on the average, from the 

 distance x + A t from the origin of the tube. There is con- 

 sequently an excess of 



particles of the first kind which in unit time cross unit area 



