﻿Molecular Motions in Thermodynamics. 63 



direction and in velocity ; therefore the particles cannot be under 

 the action of balanced forces, nor of no force. 



4. If one boundary of the limited space within which the 

 moving particles are contained is a moveable piston, that piston 

 may be at rest, or may have a uniform velocity, or a variable 

 velocity; and the condition to be fulfilled in order that the pis- 

 ton may be at rest, or may move uniformly, is, that the pressure 

 applied to its outer surface shall be equal to the pressure exerted 

 against its inner surface by the confined particles. 



5. The various investigations of the consequences of the hypo- 

 thesis of molecular motions with which I am acquainted, though 

 differing much in detail, are all identical in principle ; and I do 

 not know of any passage in the writings of any one of the 

 authors of those investigations, which is, either avowedly or im- 

 plicitly, in the slightest degree inconsistent with the dynamical 

 principles which I now quote in Mr. Heath's words : — " No 

 acceleration, and therefore no heat, is generated by the action of 

 forces which are in equilibrium ; " and " where the force meets 

 with no resistance . ... all the force is employed in producing 

 acceleration." 



6. With respect to the mode of applying those principles, the 

 following remark may seem a truism, but is nevertheless of im- 

 portance to the present discussion. In reasoning about the 

 effects of forces, the body as to whose acceleration or retardation, 

 or uniformity of motion, conclusions are drawn, ought to be the 

 same with that to which the forces are applied ; and no conclusion 

 as to the motion or rest of one body can be drawn from the fact 

 of the equilibrium or non-equilibrium of the forces applied to 

 another body. For example, if the outside and inside of a piston 

 be acted on by equal and opposite pressures, the legitimate con- 

 clusion is that the piston will remain at rest or move at a uni- 

 form speed : the equilibrium of the forces applied to the piston 

 proves nothing as to the state of motion or rest of the substance 

 that is confined by it. If, on the other hand, the outside and 

 inside of the piston be" acted on by unequal pressures, the legi- 

 timate conclusion is that the piston will be accelerated or re- 

 tarded, as the case may be ; and nothing can be concluded as to 

 the state of the particles confined. In order to determine 

 whether or to what extent these particles will be accelerated by 

 an alteration of the space within which they are confined, we 

 must consider whether there is any want of equilibrium amongst 

 the forces exerted on them by the piston and by each other. 

 Unless the particles are at rest, there cannot be equilibrium ; 

 for they are not free to move uniformly in straight lines to an 

 indefinite distance ; and their motions, if they move at all, must 

 certainly be deflected, and possibly accelerated and retarded ; 

 and such changes imply unbalanced forces. 



