166 PHENOMENA DEPENDENT ON MOLECULAR PATHS 71 



temperatures than at lower entails nothing that is at all 

 contrary to either reason or probability. For the speed of 

 the particles increases with the temperature, and, therefore, 

 also the intensity of the stress during collision, and it is 

 easily conceivable that the more strongly the particles col- 

 lide, the nearer they approach each other. 



We might perhaps suppose that the molecules become 

 looser in their joints during rise of temperature this is a 

 safe assumption at least with compounds so that one pene- 

 trates into another the more easily and deeply the warmer 

 they are. 1 Or we might assume with Stefan 2 that the 

 molecules are surrounded by atmospheres of ether which, 

 like elastic bodies, are compressed the more during a 

 collision the more intense the blow. We could, finally, share 

 Maxwell's 3 view, according to which two molecules that 

 collide are repelled from each other because, when they 

 approach very near together, they act on each other with 

 repulsive forces. On all these different hypotheses the 

 particles must come the nearer together the greater their 

 relative velocity ; or, in other words, the sphere of action 

 is the smaller the higher the temperature, and we ought 

 therefore to expect that the molecular free path increases 

 with rising temperature. This has in fact been proved, as 

 will be later shown, by measurements on viscosity and allied 

 phenomena. 



W. Sutherland 4 has attempted to give an essentially 

 different explanation of these facts. He assumes forces 

 between the gaseous particles, when very near together, 

 which are not repulsive, as Maxwell takes them, but, on 

 the contrary, attractive ; his supposition agrees, therefore, 

 the best with the known observations which Joule and 

 Lord Kelvin 5 made on the heat-phenomena of certain 

 gases streaming from a holder. These attractive forces do 



1 Pogg. Ann. 1873, cxlviii. p. 233. 



2 Wiener Sitzungsber. 1872, Ixv. Abth. 2, p. 339. 



3 Phil. Trans. 1866, clvi. p. 257 ; 1867, clvii. p. 51. Phil. Mag. 1868 [4] 

 xxxv. p. 133. Scientific Papers, ii. pp. 11, 29. 



4 Phil. Mag. 1893 [5] xxxvi. p. 507. 



5 Phil. Trans. 1853, cxliii. p. 357 ; 1854, cxliv. p. 321 ; 1860, cl. p. 325 ; 

 1862, clii. p. 579. 



