in Conceptions concerning the Constitution of Matter. 459 



or other, must enter as an element into our theory. And it is 

 also true that inertia, in the exact sense as understood by New- 

 ton, is not in all respects applicable to the theory of atoms being 

 centres of force. But if we are allowed to differ from the ordi- 

 nary views in regard to the constitution of matter, we are equally 

 at liberty to differ in regard to our views of the nature of vis 

 inertia. 



When a body is in motion its moving force is in proportion 

 to the square of its velocity. The question arises, why is the 

 motion of the body an energy ? How is it that force can be 

 stored up in the body under the form of motion ? The answer, 

 according to the ordinary view, is, because the body possesses 

 vis inertice. But this is simply saying in other words, an inert 

 body in motion is a force or energy. According to the dyna- 

 mical view the answer is equally the same ; centres of force in 

 motion are energies. In this respect vis inertice must be re- 

 garded as a quality of matter, whatever our views may be. 

 A priori it is just as natural to suppose that the motion of the 

 one should be an energy as the motion of the other. A body 

 in motion is a force or energy ; but we are in profound igno- 

 rance of the reason why it is so. It is no answer to say that a 

 body in motion is a force, because it possesses vis inertia. This 

 is merely asserting the fact, not giving the reason. We know 

 from experience that a body possesses some unknown quality, 

 by virtue of which it is, when in motion, an energy or force. 

 Newton calls this quality vis inertice, because, according to his 

 idea of matter, a body is inert, being altogether destitute of 

 active qualities. The advocate of the dynamical views, who 

 does not regard matter as wholly inert, may, if he chooses, in 

 conformity to common usages, designate this unknown quality 

 by the term vis inertice. The quality must have some name, 

 and perhaps it is as well to abide by the old one. But if we 

 imagine that when we assert that a body in motion is an energy 

 because it possesses vis inertice we convey to the mind some 

 idea how it happens that a moving body is such, we certainly 

 deceive ourselves. 



Our knowledge of vis inertice is exclusively derived from expe- 

 rience. No one could predict a priori that matter possesses 

 inertia. The advocate of the old theory has therefore no war- 

 rant whatever to assert a priori that a centre of force in motion 

 is not a force or energy by virtue of the motion. And if he has 

 no warrant a priori, he has as little a posteriori ; for how can he 

 who maintains that all matter is essentially solid, prove experi- 

 mentally that matter constituted of forces has no vis inertice ? 



Although we are unable in the present state of our know- 

 ledge to explain fully how it happens that when, for example, 



2H2 



