188 Lord Kelvin on 



not without proof be assumed to be equal. Let us therefore 

 denote by Q g N the excess of the number of G molecules 

 crossing any intermediate plane towards RRR over the 

 number crossing towards GGG, and by Q r N the excess of 

 the number of R molecules crossing towards GGG above 

 that crossing towards RRR. We have now two different 

 diffusivities of which the mean values through the whole 

 range between the bounding planes are given by the equations 



D g =Q g a ; Dr=Q r a; 



one of them, D g , the diffusivity of the green molecules, and 

 the other, D r , the diffusivity of the red molecules through 

 the heterogeneous mixture in the circumstances explained in 

 § 37. We must not now assume the gradients of density of the 

 two gases to be uniform as expressed by (1) of § 37, because 

 the homogeneousness on which these equations depend no 

 longer exists. 



§ 39. To explain all this practically*, let in the diagram 

 the planes GGG, and RRR, be exceedingly thin plates of 

 dry porous material such as the fine unglazed earthenware of 

 Graham's experiments. Instead of our green and red marked 

 molecules of the same kind, let us have two gases, which we shall 

 call G and R, supplied in abundance at the middles of the two 

 ends of a non-porous tube of glass or metal, and guided to flow 

 away radially in contact with the end-plates as indicated in 

 the diagram. If the two axial supply-streams of the two pure 

 gases are sufficiently abundant, the spaces GGG, RRR, close 

 to the inner sides of the porous end-plates will be occupied 

 by the gases G and R, somewhat nearly pure. They could 

 not be rigorously pure even if the velocities of the scouring 

 gases on the outer sides of the porous end-plates were com- 

 parable with the molecular velocities in the gases, and if the 

 porous plates were so thin as to have only two or three 



. * For a practical experiment it might be necessary to allow for the 

 difference of the proportions of the G gas on the two sides of the RRR 

 plate and of the R gas on the two sides of the GGG plate. This would 

 be exceedingly difficult, though not impossible, in practice. The 

 difficulty is analogous to that of allowing for the electric resistances of 

 the connexions at the ends of a stout bar of metal of which it is desired 

 to measure the electric resistance. But the simple and accurate 

 u potential method " b}*- which the difficulty is easily and thoroughly 

 overcome in the electric case is not available here. I do not, however, 

 put forward the arrangement described in the text as an eligible plan for 

 measuring the inter- diffusivity of two gases. Even if there were no other 

 difficulty, the quantities of the two pure gases required to realize it would 

 be impracticably great. 



