﻿Manchester Memoirs, Vol. xlix. (1905), No. i> 13 



it is believed that the heat of a substance is occasioned 

 by the mean speed at which the molecules of which it is 

 composed are travelling. To deal with the three dif- 

 ferences between our two aspects of things in turn ; it is 

 evident first that, while our ignorance of the speed of an 

 individual molecule is so great that we try to conceal it 

 by saying that it is determined by chance, our knowledge 

 of the average speed of myriads of them is so accurate 

 that certain laws of thermo-dynamics have been formu- 

 lated. Secondly, it has been calculated that a curve 

 representing the frequency of the various speeds spread 

 over the molecules is an ordinary curve of error ; so that 

 although that which is true of the mass is also true of 

 some of the molecules, it is by no means true of all of 

 them. Thirdly, the only point of view from which we 

 can regard this phenomenon at present is that from which 

 we can only discern the mass-result : but that it is by no 

 means inconceivable that there may be another point of 

 view is evident to all who are familiar with Clerk 

 Maxwell's demon, a being who is so essential to my 

 argument that I shall make no apology for quoting his 

 creator's description of him in full : " One of the best- 

 established facts in thermo-dynamics is that it is impos- 

 sible in a system enclosed in an envelope which permits 

 neither change of volume nor passage ot heat, and in 

 which both the temperature and pressure are everywhere 

 the same, to produce any inequality of temperature or 

 pressure without the expenditure of work. This is the 

 second law of thermo-dynamics, and it is undoubtedly 

 true so long as we can deal with bodies only in mass, and 

 have no power of perceiving or handling the separate * 

 molecules of which they are made up. But if we con- 

 ceive a being whose faculties are so sharpened that he 

 can follow evey molecule in its course, such a being, ■ 



* My italics. 



