88 MOLECULAR MOTION AND ITS ENERGY 38 



gypsum, charcoal, silica, and burnt magnesium. Neumann 

 aikd Feddersen called the phenomenon thermo-diffusion. 

 / Osborne Reynolds, 1 who used the name thermal 

 transpiration, repeated Feddersen's experiments, and 

 made a great number of actual measurements of the 

 difference of pressure that was produced at the warmer 

 side of the partition. In the above-mentioned memoir 

 Maxwell has given a very simple explanation of the 

 phenomenon on the basis of the kinetic theory of gases. 

 His theory starts from the assumption that the number 

 of particles of gas which collide with the walls of the 

 containing vessel in unit time is proportional not only to 

 the number N of molecules contained in unit volume but 

 also to their mean speed fl ; and, when the area struck 

 is taken equal to unity, this number is expressed by the 

 product 



as in a formula developed in 37. 



If in unit volume on one side of the partition there are 

 jVj molecules with the mean speed flj, and on the other JV 2 

 molecules with the mean speed I1 2 , a unit area of the narrow 

 openings in the wall will be met on one side by 



molecules, and on the other side by 



#A. 



If then 



more molecules will pass over in the first direction than in 

 the second, but fewer if 



There consequently ensues a flow of gas from that side on 

 which the product Nfl has the greater value towards that 

 where the value is the smaller. 



Suppose now, first of all, as is the case at the beginning 

 of an experiment, that there is the same pressure on both 



1 Phil. Trans, clxx. pt. 2, 1879, p. 727; Wied. Beibl. vi. 1882, p. 455. 



