180 On the Mechanical Theory of Crookes's Force. 



it — and that the force does not arise from the difference in 

 temperature in the two opposite plates, but from the difference 

 in temperature of the two surfaces of the same plate. 



It appears, then, that such a separating force as that assumed 

 by Mr. Fitzgerald would not explain the phenomena in ques- 

 tion, and therefore that these phenomena afford us no ground 

 for assuming the existence of such a force. Any reason there 

 may be for assuming such a force must therefore come out of 

 some hypothesis as to the constitution of gas ; and in this 

 respect the result of Mr. Fitzgerald's investigation is not very 

 conclusive. 



Adopting the hypothesis of Clausius, Mr. Fitzgerald's rea- 

 soning leads him to the conclusion that there is no such separa- 

 ting force (see the bottom of page 22). Instead, however, of ac- 

 cepting the conclusion, Mr. Fitzgerald concludes that Clausius 

 is wrong : — " It seems certain that the hypothetical distribution 

 Clausius assumed is not at all adequate to represent the actual 

 one." He then proceeds to modify the expression derived 

 from Clausius' hypothesis so as to make it yield the force for 

 which he is looking; but he attempts no explanation or exami- 

 nation of the physical meaning of such a modification ; so 

 that admitting, as I have pointed out, that we have no expe- 

 rimental evidence of such a separating force, Mr. Fitzgerald's 

 investigation clearly affords us none, but, on the other hand, 

 shows either that Clausius is wrong or that there is no such 

 force. 



Had Mr. Fitzgerald been true to his mathematics, had he 

 accepted the conclusion that there is no such separating force 

 as he has assumed, and then examined the physical meaning 

 of the modifications of the expressions which he has introduced, 

 I venture to say that he would have found that his modi- 

 fications of Clausius' hypothesis, on the assumption that the 

 direction of flow of heat is everywhere the same, would corre- 

 spond with the true expressions of Clausius' hypothesis when 

 there is divergence in the directions along which heat is 

 flowing. 



This divergence turns out to be an essential condition in 

 order that there may be force such as that which causes mo- 

 tion in the arms of the radiometer. And since the pressure in 

 the direction of flow is greater or less than the mean pressure, 

 according as the lines of flow diverge or converge, the pressure 

 will be greater against the hot side of a plate and less against 

 the cold side. 



When I first suggested that there would be an inequality 

 of gaseous pressure arising from a communication of heat, I 

 had not realized that, besides depending on the quantity of 



