and the Conservation of Energy. 39 



of force, acting upon each other according to laws which do 

 not vary with time but do vary with distance." 



This conception of matter is of course an old one, being 

 that usually known by the name of Boscovitch. It has not, 

 however, been generally accepted by writers on Mechanics ; 

 and in recent times certain special objections have been raised 

 against it, which it seems well briefly to consider. 



1. An objection, due to Professor Maxwell*, is that the con- 

 ception does not comprise the idea of inertia, which is a fun- 

 damental fact with regard to matter. But when we say that 

 a body has inertia, we simply mean that a finite force, acting 

 upon it for a finite time, generates only a finite velocity. 

 Hence it follows that any body we can see or feel, or know and 

 investigate in any way, must have inertia; for suppose a body 

 to possess no inertia, then the first time any force was applied 

 to it, it would at once be removed to an indefinite distance, 

 and would therefore be beyond the reach of investigation. To 

 say that matter has inertia is therefore merely to assert the 

 general principle that any thing our senses can deal with must 

 be finite; and it is therefore a condition anterior to any theory 

 of matter, not a part of such theory. 



2. An objection given by Lamef is that bodies, and espe- 

 cially homogeneous crystals, are not, within the limits of 

 observation, denser at the centre than they are at the surface, 

 which on the theory of central forces they apparently should 

 be. But on this it may be observed as follows. 



It may be admitted that collections of centres of force, at 

 rest under their mutual actions, would be more dense towards 

 the centre. We know no such bodies in nature. The nearest 

 approach to it is the case of bodies so large that their mole- 

 cular motions, and also their want of homogeneity, may be 

 neglected in comparison of their mass. But the condition of 

 large bodies does appear to agree with the theory ; e. g. even 

 the mean specific gravity of the earth (5*6) is greater than that 

 of all bodies, except a few metals, at the surface. The want of 

 homogeneity can have little influence at the temperature and 

 pressure which prevail in the interior. 



Again, it is known that, in all bodies, the actual centres of 

 force must be bound up together in molecules so closely as to 

 form coherent wholes, which no known force can change or 

 break up. The relations in a crystal therefore are not those 

 among separate centres of force, but among separate molecules. 



Again, these molecules, being hot, are in rapid and conti- 

 nuous motion. 



* ' Theory of Heat,' p. 85. 



t Elasticity des Coi-ps solides, p. 333. 



