350 ON THE CONSERVATIOX OF FORCE. 



substance, for its quantity is not unchangeable. It can 

 be produced anew from the vis viva of motion destroyed ; 

 it can be destroyed, and then produces motion. We must 

 rather conclude from this that heat itself is a motion, an 

 internal invisible motion of the smallest elementary par- 

 ticles of bodies. If, therefore, motion seems lost in 

 friction and impact, it is not actually lost, but only passes 

 from the great visible masses to their smallest particles ; 

 while in steam-engines the internal motion of the heated 

 gaseous particles is transferred to the piston of the 

 machine, accumulated in it, and combined in a resultant 

 whole. 



But what is the nature of tliis internal motion, can only 

 be asserted with any degree of probability in the case of 

 gases. Their particles probably cross one another in 

 rectilinear paths in all directions, until, striking another 

 particle, or against the side of the vessel, they are re- 

 flected in another direction. A gas would thus be 

 analogous to a swarm of gnats, consisting, however, of 

 particles infinitely small and infinitely more closely 

 packed. This hypothesis, which has been developed by 

 Kronig, Clausius, and Maxwell, very well accounts for all 

 the phenomena of gases. 



What appeared to the earlier physicists to be the con- 

 stant quantity of heat is nothing more than the whole 

 motive power of the motion of heat, which remains con- 

 stant so long as it is not transformed into other forms of 

 work, or results afresh from them. 



We turn now to another kind of natural forces which 

 can produce work — I mean the chemical. We have to- 

 day already come across them. They are the ultimate 

 cause of the work which gunpowder and the steam-engine 

 produce ; for the heat which is consumed in the latter, 

 for example, originates in the combustion of carbon — 

 that is to say, in a chemical process. The burning of 



