496 THE CONSERVATION OF ENERGY 



If there be a system of say two particles revolving about each 

 other, under their mutual attraction they have a definite amount of 

 energy, actual and potential. Now, let a third particle come within 

 the sphere of attraction of either particle, it will increase the energy of 

 the system ; there is no limit to the number of particles which might 

 not become similarly incorporated with the system. Thus energy 

 would be continually created by the chance proximity of other par- 

 ticles. We may perhaps get out of this difl&culty by supposing the 

 spheres of attraction of every particle to include all other particles, 

 but we shall hereafter see that a supposition of this kind, which as 

 far as our observation goes can only be made in the case of the attrac- 

 tion of gravitation, will be of no service. 



Sir John Herschel, although a believer in action at a distance,, 

 shows very clearly, in a lecture on the " Origin of Force," that the 

 principle of conservation, regarded merely as asserting the periodic 

 restorations of actual or dynamic energy, cannot, from the very nature 

 of the case, be proved — that, in fact, it is almost inconceivable that 

 it should express a law of nature. He could be led to no other result, 

 holding as he did the theory of action at a distance. 



We now come to the theory of action by contact. The first ques- 

 tion naturally asked is, what is understood by the term contact. 

 Contact is to be taken in the same sense as in geometry. Two 

 geometrical figures are in contact when adjacent surfaces, lines, or 

 points, as the case may be, are coincident, and there is at the same 

 time no space of thi'ee dimensions common to both. What is force 

 according to this theory 1 This question may be answered thus : 

 when two atoms are in motion, so that each would occupy a certain 

 space at a certain time if the other did not exist, they will come in 

 contact ; and on account of their impenetrability, or, in other words, . 

 since two bodies cannot occupy the same space at the same time, their 

 motions must be changed, and to this action the name force is given. 

 It is simply collision or impact. It is measured in the usual way by 

 the changes of momentum and direction, referred to certain space . 

 considered fixed. 



Sir John Herschel, in the same lecture referred to above, shows 

 most conclusively that the dynamic energy of a system constituted 

 thus must infallibly diminish, and that at length a state will be 

 reached when all relative motion wUl be destroyed ; or that, where it 

 does exist, the moving parts will fly oflf into space, never to meet 



