and the Conservation of Energy. 531 



(Maxwell's or those of the Germans) enters most closely into 

 the actual nature of the phenomena, and is the nearest ap- 

 proach towards the ultimate solution of the problem. 



Now, my first contention is this, that any one who admits 

 my statement of the conservation of energy*, and also the 

 ordinary statement of the equality of action and reaction, is 

 bound to admit my conclusion. 



And my second contention is that, whether my statement 

 is identical with that of the "text-books" or not, it is a 

 reasonable and true statement, that it is just as axiomatic as 

 any other, and that it ought to be accepted. 



This second contention, however, is distinctly open to dis- 

 cussion. 



Now, I have pointed out in section 84 of my little book on 

 Mechanics, and on page 280 of the Phil. Mag. for 1879, that 

 the two fundamental forms of energy correspond to the two 

 factors in the product work. A body exerting force pos- 

 sesses energy, and a body moving through space possesses 

 energy, the first form being called potential, or dynamic, or 

 static, the second being called kinetic ; but a body is not 

 doing work unless it is both exerting force and moving through 

 space ; and in this case it is losing energy and transferring 

 it to the body upon which the work is being done, the energy 

 at the same time being always transformed from kinetic to 

 potential, or vice versa. 



Consider, for instance, the earth and a stone (or any other 

 " attracting " bodies) supposed at rest in space, and separated 

 from each other against whatever it be that is called their 

 mutual attraction. A common mode of stating the condition 

 of affairs is to state that the stone possesses potential or pos- 

 sible energy, which will become actual when it is allowed to 

 fall, the sum of the possible and actual energies remaining 

 constant during the whole time of fall, until, in fact, they are 

 knocked out of the body by the blow. This, however, is the 

 very commonest mode of stating the matter ; and I suppose 

 nearly every one will agree that the phrases actual and possible 

 energy are little better than nonsense, or, at any rate, that 

 any law of conservation of energy founded upon such notions 

 must be utterly meaningless. 



* Phil. Mag. Jan. 1881, p. 36, and Oct. 1870, p. 278. It may be re- 

 peated for convenience — The energy of a body is increased whenever -work 

 is done upon it, and diminished whenever work is done by it, by an 

 amount in each case numerically equal to the work done ; but whenever 

 two bodies act on each other the work done by the one is identical with 

 that done upon the other ; in other words, energy is never generated or 

 destroyed by such actions, but is simply transferred from one body to the 

 other. 



