164 EFFECT OF VARIOUS PROCESSES. 



Yet while we exclude the kind of action which we call ther- 

 mal between the fluid and the containing vessel, we allow the 

 kind which we call work in the narrower sense, which takes 

 place when the volume of the fluid is changed by the motion 

 of a piston. This agrees with what we have supposed in 

 regard to the external coordinates, which we may vary in 

 any arbitrary manner, and are in this entirely unlike the co- 

 ordinates of the second ensemble with which we bring the 

 first into connection. 



When heat passes in any thermodynamic experiment between 

 the fluid principally considered and some other body, it is 

 actually absorbed and given out by the walls of the vessel, 

 which will retain a varying quantity. This is, however, a 

 disturbing circumstance, which we suppose in some way made 

 negligible, and actually neglect in a theoretical discussion. 

 In our case, we suppose the walls incapable of absorbing en- 

 ergy, except through the motion of the external coordinates, 

 but that they allow the systems which they contain to act 

 directly on one another. Properties of this kind are mathe- 

 matically expressed by supposing that in the vicinity of a 

 certain surface, the position of which is determined by certain 

 (external) coordinates, particles belonging to the system in 

 question experience a repulsion from the surface increasing so 

 rapidly with nearness to the surface that an infinite expendi- 

 ture of energy would be required to carry them through it. 

 It is evident that two systems might be separated by a surface 

 or surfaces exerting the proper forces, and yet approach each 

 other closely enough to exert mechanical action on each other. 



