1879.] 



Properties of Matter in the Gaseous State. 



307 



TJie Sufficiency of the Demonstration that Gas is not Structureless. 



6. In order to prove that gas is not structureless, it is not necessary 

 that we should be able to perceive the actual structure ; we have only 

 to find some property of a certain quantity of gas which can be shown 

 not to be possessed by all the parts, some property which is altered 

 by a rearrangement of the parts. 



Hitherto I believe that no such property has been recognised, or, at 

 all events, the conclusions to be drawn from such a property have not 

 been recognised. The phenomena of transpiration, as well as those of 

 the radiometer, depend on such properties, but these properties have 

 not been sufficiently understood to bring out the conclusion. This 

 conclusion, however, follows directly from the law indicated in Article 4, 

 viz., that the results of transpiration depend on the relation between 

 the size of the passages and the density of the gas. 



The Results Deduced from Theory. 



7. Although the existence of the phenomena of thermal transpira- 

 tion, and the existence of the law of corresponding results at corres- 

 ponding densities, have been verified by experiment, they were not so 

 discovered ; they followed from what appeared to be a successful 

 attempt to complete the explanation which I had previously offered of 

 the forces which result when heat is communicated from a surface to 

 a gas,* and the phenomena of the radiometer. 



Having found, what I had not at first perceived, that according to 

 the kinetic theory the excess of pressure resulting from the communi- 

 cation of heat to a gas must depend on . the fact of the surface from 

 which the heat flows being of limited extent, and must follow a law 

 depending on some relation between the mean path of a molecule 

 and the size of the surface, it appeared that by using vanes of 

 comparatively small size the force should be perceived at correspond- 

 ingly greater pressures of gas. 



On considering how this might be experimentally tested, it appeared 

 that to obtain any result at measurable pressures the vanes would 

 have to be very small indeed ; too small almost to admit of experiment. 

 And it was while searching for some means to obviate this difficulty 

 that I came to perceive that if the vanes were fixed, then instead of 

 the movement of the vanes we should have the gas moving past the 

 vanes — a sort of inverse phenomenon ; and then instead of small vanes 

 small spaces might be allowed for the gas to pass. Whence it was at 

 once obvious that in the porous plugs I should have the means of 

 verifying these conclusions. I followed up the idea, and by a method 

 which I devised of extending the dynamical theory of gases, so as to 

 take into account the forces (tangential and normal) arising from a 



* " Proc. Eoy. Soc," vol. xxii, p. 402, and " Phil. Trans.," vol. clxvi, p. 726. 



