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XXXII. On Mr. Gr. F. Fitzgerald's Paper " On the Mechanical 

 Theory of Crookes's Force." By Osborne Reynolds, 

 F.B.S* 



MR. FITZGERALD appears to have overlooked the fact 

 that my paper in the Proceedings of the Royal Society, 

 1874, " On the Surface Forces caused by Evaporation and 

 Condensation," was published more than a year before Mr. 

 Crookes published any account of the radiometer ; otherwise 

 he certainly would not have fallen into the error of supposing 

 that I had concluded that the motion of the arms of the radio- 

 meter was mainly due to evaporation and condensation. That 

 such actions cannot explain continuous motion is at once ob- 

 vious. But then Mr. Fitzgerald fails to notice that on the first 

 page of my paper an experiment is described which proves this 

 very point ; and he also fails to notice that all the phenomena 

 I have considered in any way due to evaporation and conden- 

 sation were essentially intermittent. 



It would appear that Mr. Fitzgerald has not read my paper; 

 for after stating that the method by which I " tried to show 

 that a surface, when communicating heat to gas, is subject to 

 an increased pressure is open to the overwhelming objection 

 that this increased pressure would be almost instantaneously 

 transmitted to all parts of the enclosed gas," he devotes some 

 fourteen pages to the attempt to prove the very same thing. 



To point out these errors in Mr. Fitzgerald's statements 

 constituted my main object in writing this note ; but I would 

 say a few words on the subject in question and Mr. Fitz- 

 gerald's treatment of it. 



Mr. Fitzgerald bases his theory on Mr. Stoney's view that 

 the phenomena of the radiometer are to be explained by the 

 fundamental assumption " that when two surfaces at different 

 temperatures are in presence of one another with a gas between 

 them, there exists a force terding to separate them." Assu- 

 ming that it is here meant that the gas should surround the 

 surfaces and not merely exist between them, it may appear at 

 first sight as though this assumption would explain the pheno- 

 mena ; but on closer examination it will appear, as I have pre- 

 viously pointed out, that this is not the case. Under such 

 conditions as are assumed the experiments show that the force 

 would not tend to separate the surfaces, but such forces as 

 there might be would impel both surfaces in the same direc- 

 tion — showing that the force does not act between the two sur- 

 faces, but between each surface and the gas which surrounds 



* Communicated Iby the Author. 



