III. HEAT. 



IF we now take HEAT as our starting point, we shall find 

 that the other modes of force may be readily produced by 

 it. To take motion first : this is so generally, I think I may 

 say invariably, the immediate effect of heat, that we may almost, 

 if not entirely, resolve heat into motion, and view it as a 

 mechanically repulsive force, a force antagonist to attraction 

 of cohesion or aggregation, and tending to move the particles 

 of all bodies, or to separate them from each other. 



It may be well here to premise, that in using the terms 

 1 particles ' or ' molecules,' which will be frequently employed 

 in this Essay, I do not use them in the sense of the atomist, 

 or mean to assert that matter consists of indivisible particles 

 or atoms. The words will be used for the necessary purpose 

 of contradistinguishing the action of the indefinitely minute phy- 

 sical elements of matter from that of masses having a sensi- 

 ble magnitude, much in the same way as the term ' lines ' or 

 4 points ' may be used, and with advantage in an abstract 

 sense ; though there does not exist, in fact, a thing which has 

 length and breadth without thickness, and though a thing with- 

 out parts or dimensions is nothing. 



If we put aside the sensation which heat produces in our 

 own bodies, and regard heat simply as to its effects upon in- 

 organic matter, we find that, with a very few exceptions, which I 



