ACTION OF HEAT. 147 



of it is no proof that it is not there, any more than 

 our ignorance of any other truth is a contradiction 

 of that. The different susceptibilities of different 

 substances to heat are the means by which almost 

 every change is performed, not only in nature but 

 in the arts ; and even when we cut wood with a 

 knife, or grind iron upon a stone, it is by no means 

 improbable that we effect our purpose chiefly, if not 

 wholly, through the instrumentality of heat. When 

 we work hard, the tool gets heated in its whole sub- 

 stance ; and when a blacksmith has no other means 

 of lighting his forge, he has only to hammer a piece 

 of iron on the anvil till it be red-hot, and thrust it 

 into the coals, and he instantly has a fire. Even 

 when we move our bodies the parts moved become 

 heated, nor can we get any instance in which mo- 

 tion is not accompanied by heat, and heat by motion ; 

 and if there be enough of heat, there is light along 

 with it. Sometimes indeed we are sensible of the 

 one of these, and not of the other two ; and some- 

 times we are sensible of any two of the three, and 

 not of the remaining one ; but though, in all cases, 

 our senses are our evidence of that which they do 

 reveal to us, they can in no instance be evidence 

 of that which they do not reveal. 



The general action of heat, both in nature and in 

 art, is thus to separate the particles of simple bodies, 

 and the parts of compound ones ; and there can be 

 no separation without motion, whether that motion 

 be such as we can divide in succession, and by that 

 means observe, or not. 



In bodies which are simple, or in compounds the 

 parts of which are equally sensible to the action of 

 heat, heat merely softens, melts, and converts into 

 vapour. But the heat does not proceed uniformly 

 in its action : there is one point at which the sub- 

 stance becomes liquid, and another at which it passes 

 into the state of air or vapour ; and in each of its 

 three states it can, generally speaking, bear a cer- 



