and Radiometer Motion. 147 



causes conduction, there are also involved the action of the 

 lateral components of motion in bringing the molecules up 

 to the surface, and the action of the surface in equalizing the 

 opposite momenta of the opposite groups. 



Bearing these incidents and steps in the history of the 

 kinetic theory in mind the precise errors into which Mr. 

 Sutherland has fallen become evident. In the paragraph 

 beginning in middle of p. 375 he begins : " In the kinetic 

 theory the molecules which are considered characteristic of 

 an element are those which have experienced collision in it." 



Then he gives reasons for thinking that the walls of the 

 tube produce no effect on the characteristics of the molecules 

 which rebound from them. And thus having cut away the 

 only cause of thermal transpiration he proceeds : — 



" Thus, then, if we do not have to take account of reflexion 

 from the walls of the tube, we can consider the gas in it as 

 part of an indefinite mass such that the temperature through- 

 out a plane perpendicular to the axis is the same as that in 

 the section of the tube made by the plane. We wish to find 

 the number of molecules crossing any section of the tube. 

 This is done by Clausius in his theory of conduction in gns^s, 

 and with greater refinements of accuracy by Tait ; but for 

 the sake of clearness we will make the calculation here to a 

 degree of accuracy suitable for present requirements/' 



It is to be noticed here that he has not stated whether or 

 not the infinite mass of gas is at rest or in motion with respect 

 to the plane; but in the next paragraph he proceeds to con- 

 sider the quantity of matter carried across the plane by each 

 of the opposite groups of molecules, having respectively the 

 mean characteristics of uniform gas at rest in the mean con- 

 dition of the actual gas at a distance from the plane on either 

 side equal to the half of the mean component of the free path — 

 thus defining the gas as at rest ; and at the same time showing 

 that he has in no way realized the point of Clausius's paper, 

 which he quotes — showing, in fact, that he cannot even have 

 read it. Having thus fallen into the error from which 

 Clansius released Maxwell, the inevitable result is that he 

 arrives at the absurd conclusion that in a gas at rest more 

 molecules cross the fixed plane from the cold side than from 

 the hot side. But instead of recognizing the absurdity of 

 this conclusion, he says — " which amounts to the same thing 

 as if the gas had a velocity/' and then proceeds to make this 

 unfounded and self-induced velocity the foundation of thermal 

 transpiration, introducing a hypothetical traction between the 

 gas and the solid surface, the effect of which he had previously 

 shown to be nothing. 



As shown (art. 88) in my paper, the velocity thus erro- 



