306 ON THE CONSERVATION OF FORCE. 



With atmospheric air . -. . 426-0 metres 



oxygen , , . . . 425-7 



nitrogen .. ... . ' . . . 431-3 



hydrogen . ... . . 425*3 



Comparing these numbers with those which determine the 

 equivalence of heat and mechanical work in friction, as close an 

 agreement is seen as can at all be expected from numbers which 

 have been obtained by such varied investigations of different 

 observers. 



Thus then : a certain quantity of heat may be changed into 

 a definite quantity of work ; this quantity of work can also be 

 retransformed into heat, and, indeed, into exactly the same 

 quantity of heat as that from which it originated; in a me- 

 chanical point of view, they are exactly equivalent. Heat is a 

 new form in which a quantity of work may appear. 



These facts no longer permit us to regard heat as a 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 particles 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 this 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 reflected in another direction. A gas 

 would thus be analogous to a swarm of gnats, consisting, how- 

 ever, of particles infinitely small and infinitely more closely 

 packed. This hypothesis, which has been developed by Krb'nig, 

 Clausius, and Maxwell, very well accounts for all the phenomena 



What appeared to the earlier physicists to be the constant 



