CONCLUDING EEMAEKS. 191 



a similar expression for these forces : but at present the bod- 

 ies in which this parity of force has been discovered, though 

 in themselves numerous, are small compared with the excep- 

 tions, and, therefore, this point can only be indicated as prom- 

 ising a generalisation, should subsequent researches alter our 

 knowledge as to the elements and combining equivalents of 

 matter. 



With regard to what may be called dynamic equivalents, 

 i. e. the definite relation to time of the action of these varied 

 forces upon equivalents of matter, the difficulty of establish- 

 ing them is still greater. If the proposition which I stated 

 at the commencement of this paper be correct, that motion 

 may be subdivided or changed in character, so as to become 

 heat, electricity, &c., it ought to follow that when we collect 

 the dissipated and changed forces, and reconvert them, the 

 initial motion, minus an infinitesimal quantity affecting the 

 same amount of matter with the same velocity, should be re- 

 produced, and so of the changes in matter produced by the 

 other forces ; but the difficulties of proving the truth of this 

 by experiment will, in many cases, be all but insuperable ; 

 we cannot imprison motion as we can matter, though we may 

 to some extent restrain its direction. 



The term perpetual motion, which I have not unfrequent- 

 ly employed in these pages, is itself equivocal. If the doc- 

 trines here advanced be founded, all motion is, in one sense, 

 perpetual. In masses whose motion is stopped by mutual 

 concussion, heat or motion of the particles is generated ; and 

 thus the motion continues, so that if we could venture to extend 

 such thoughts to the universe, we should assume the same 

 amount of motion affecting the same amount of matter forever. 

 Where force opposes force, as in cases of static equilibrium, 

 the balance of pre-existing equilibrium is affected, and fresh 

 motion is started equivalent to that which is withdrawn into 

 a state of abeyance. 



But the term perpetual motion is applied, in ordinary par- 



