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 inva- 

 riably, 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 antagonistic 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 

 ' 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. 

 By many who adopt the atomic doctrine the term ( molecule ' 

 is used to signify a definite congeries of atoms, forming an 

 integral constituent of -matter, somewhat as a brick may be 

 said to be a congeries of sand atoms, but a structural element 

 of a house. The word ' molecule ' will be used by me for the 

 necessary purpose of contradistinguishing the action of the 

 indefinitely minute physical elements of matter from that of 

 masses having a sensible magnitude, much in the same way as 

 the terms ' lines ' or ' points ' are used in an abstract sense ; 

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

 and breadth without thickness, and though a thing without 

 parts or dimensions is nothing. 



If we put aside the sensation which heat produces in our 

 own bodies, and regard heat simply in relation to its effect 

 upon inorganic matter, we find that, with a very few excep- 

 tions, which I shall presently notice, the effects of what is 

 called heat are simply an expansion of the matter acted upon, 

 and that the matter so expanded has the power by its own 

 contraction of communicating expansion to all bodies in conti- 



