168 Physical Properties [CH. vn 



distinguishable arrangement of coloured inks. If, however, we start by stirring 

 a uniform violet ink composed of a mixture of red and blue inks, then it 

 is possible, although not probable, that the effect of the stirring will be to 

 separate the inks of different colour, so that one half of the vessel is occupied 

 solely by red, and the other solely by blue ink. And from the dynamical 

 standpoint it is no less probable that this should occur, than that we should 

 be able to start stirring inks which were separated initially as regards colour. 



195. With reference to this subject, some well-known remarks of 

 Maxwell * are of extreme interest. He says : " One of the best established 

 facts in thermodynamics is that it is impossible in a system enclosed in an 

 envelope which permits neither change of volume nor passage of heat, and in 

 which both the temperature and the pressure are everywhere the same, to 

 produce any inequality of temperature or of pressure without the expenditure 

 of work. This is the second law of thermodynamics, 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 conceive a being whose faculties are so sharpened that he can 

 follow every molecule in its course, such a being, whose attributes are still as 

 essentially finite as our own, would be able to do what is at present im- 

 possible to us. For we have seen that the molecules in a vessel full of air at 

 uniform temperature are moving with velocities by no means uniform though 

 the mean velocity of any great number of them, arbitrarily selected, is almost 

 exactly uniform. Now let us suppose that such a vessel is divided into two 

 portions A and B, by a division in which there is a small hole, and that a being, 

 who can see the individual molecules, opens and closes this hole, so as to allow 

 only the swifter molecules to pass from A to B, and only the slower ones 

 to pass from B to A. He will thus, without expenditure of work, raise the 

 temperature of B and lower that of A, in contradiction to the second law of 

 thermodynamics." 



Thus Maxwell's sorting demon could effect in a very short time, what 

 would probably take a very long time to come about if left to the play of 

 chance. There would, however, be nothing contrary to natural laws in the 

 one case any more than in the other. 



The reader who wishes to study the question of irreversibility further is 

 referred to the following works:' 



(i) "Report on the Present State of our knowledge of Thermodynamics, 

 specially with regard to the Second Law," by J. Larmor and G. H. Bryan, 

 British Association Report, 1891 (Cardiff), p. 85. 



(ii) Elementary Principles of Statistical Mechanics, J. Willard Gibbs 

 (Scribners, New York), 1902. 



(iii) Vorlesungen ilber Gastheorie, Boltzmann. 



* Theory of Heat, p. 328. 



