336 JAMES CLERK MAXWELL. [CHAP. XL 



a man as a mere machine, and find that neither a perfect 

 heat engine nor an electric engine could produce so much 

 work and waste so little in heat. We therefore save our 

 pains in investigating any theories of animal power based on 

 heat and electricity. We see also that the soul is not the 

 direct moving force of the body. If it were, it would only 

 last till it had done a certain amount of work, like the 

 spring of a watch, which works till it is run down. The 

 soul is not the mere mover. Food is the mover, and 

 perishes in the using, which the soul does not. There is 

 action and reaction between body and soul, but it is not of 

 a kind in which energy passes from the one to the other, 

 as when a man pulls a trigger it is the gunpowder that 

 projects the bullet, or when a pointsman shunts a train it is 

 the rails that bear the thrust. But the constitution of our 

 nature is not explained by finding out what it is not. It is 

 well that it will go, and that we remain in possession, though 

 we do not understand it. 



Hr. Clausius of Zurich, one of the heat philosophers, has 

 been working at the theory of gases being little bodies flying 

 about, and has found some cases in which he and I don't 

 tally, so I am working it out again. Several experimental 

 results have turned up lately, rather confirmatory than 

 otherwise of that theory. 



I hope you enjoy the absence of pupils. I find that the 

 division of them into smaller classes is a great help to me 

 and to them; but the total oblivion of them for definite 

 intervals is a necessary condition of doing them justice at 

 the proper time. 



To FLEEMING JENKIN, Esq. 1 



27th Aug. 1863. 



., . . To compare electromagnetic with electrostatic 

 units : 



1st, Weber's method. Find the capacity of a condenser 

 in electrostatic measure (meters). 



1 Now Professor of Engineering in Edinburgh. 



