Properties of Matter in the Gaseous State. 341 



ence of pressure in different directions ; and, of course, I have 

 known that such must be the case since the time that I have 

 seen and proved by experiment that this direct variation of the 

 pressure depends on the convergence of the lines of flow, which 

 was before the letter referred to appeared in ' Nature.' But 

 what I have consistently maintained is, that a difference of 

 pressure in different directions (i. e. parallel and normal to 

 the hot and cold surface) will not explain the experimental 

 results : and this was the theory advanced in opposition to 

 mine, and which Mr. Fitzgerald still seems inclined to defend. 

 I am asked to mention the result which is referred to in art. 54. 

 I can only point to every phenomenon of the radiometer ; for 

 there the gas between the hot and cold surfaces always main- 

 tains a greater pressure on the hot than on the cold plate — a 

 result which is fully explained in art. 129, as the consequence 

 of the divergence of the lines of flow from the hot plate and 

 their convergence onto the cold plate, shown in fig. 13. If 

 Mr. Fitzgerald will only study the phenomena, he will see 

 that it is he who has misapprehended the entire problem. He 

 says a difference of pressure in different directions might tend 

 to cause the plates to recede from each other. Obviously it 

 would ; but then there is not the slightest evidence that the 

 plates do so tend to recede, while they actually move in the 

 same direction, the cold plate following the hot. Hence no 

 force merely causing them to separate can explain the pheno- 

 mena. I have pointed this out over and over again, and now, 

 so far from having changed my views, I have to go over the 

 same ground again. I will take a simple case — a light mill 

 with two equal radial vanes in the same plane, and on oppo- 

 site sides of the pivot, one black and one white. Let the 

 light be placed exactly opposite the vanes, and let the vanes 

 be at rest. Also let the surface of the vessel and the gas 

 be generally at the mean temperature of the vanes. If, then, 

 the force were only such as tends to separate the hot and 

 cold surfaces, there would be exactly the same force between 

 the comparatively hot black vane and the colder glass as 

 between the comparatively hotter glass and the colder white 

 vane ; for there are the same differences of temperature ; and 

 therefore the forces on the two vanes would tend to turn the 

 mill in opposite directions, and the mill would remain at rest, 

 instead of whirling round as it actually does. That the flow 

 of heat caused the surfaces to follow each other was proved 

 from the first by the experiments ; and that there is no force 

 causing the surfaces to separate of the same order of magni- 

 tude as the force which causes them to follow is now proved 

 by the kinetic theory. 



