430 The Rev. J. M. Heath on the Principles of Thermodynamics. 



dynamist to distinguish the forces which have done mechanical 

 work into two groups — those which have accelerated molecular 

 motions, and those which have only " stored up energy " in some 

 previously exhausted magazine. The words are mine and not his ; 

 but I have stated the proposition in such a form that Ithinkhewill 

 still acknowledge it as his own, while on my side I can also assent 

 to it, which I could not do in the form he had given to it. So far, 

 then, I believe there is entire agreement between us. It is in the 

 next step taken towards making this distinction among the forces 

 that I begin to dissent from him and, I fear, from the unanimous 

 opinion of all scientific men. That step is the adoption, as the 

 rule required, under the name of the first law of thermody- 

 namics, of the condition that it is those forces that " do work/' 

 according to a certain technical definition of that term, which 

 are considered as those which "accelerate molecular motion;" 

 and, of course, by necessary consequence, those which do no work 

 are those which produce no motion, but " store up energy." 

 The counter assertion, which I rely upon being able to sustain, 

 is that those forces only are employed in generating motion 

 which, according to the definition of work, do no work — and that 

 those which do work generate no motion or heat, but do " store 

 up energy ." 



Before proceeding to substantiate these opinions, it is as well, 

 to prevent misunderstanding, that I should say what I under- 

 stand that definition of work done to be, against which I am pro- 

 testing. I understand, then, that work is done, according to 

 this definition, by a force P, when, in acting upon a body m, it 

 meets with a resistance Q equal and opposite to itself, which it 

 " overcomes n by driving mina direction opposite to the action 

 of Q through a space 8v without increasing its vis viva. And 

 the measure of the work so done isfQdv, or, for the sake of 

 simplicity, let us say QSv. If I have misstated this definition, 

 I shall be sincerely grateful to Mr. Rankine or any one else who 

 will point out my error, or show me where a better one has been 

 given. It is against this definition only that I contend ; and if 

 Mr. Rankine disowns its correctness, I am and have long been 

 fighting against a mere shadow. But I shall proceed to examine 

 it on the supposition that it is a correct description of what is 

 meant by the work done by P. 



The general equation of vis viva, simplified as above for our 



mv 

 purposes, is this, (P— Q)$v= -~- . This gives us the vis viva 



-— - that is generated in m when urged through a space Sv 

 by the action of a force P against a resistance Q. If P is 



