ACTION OF HEAT. 143 



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 motion 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, whe- 

 ther that motion be such as we can divide in suc- 

 cession, 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 uni- 

 formly in its action : there is one point at which 

 the substance 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 certain range ; though there are cases in 

 which the melting and the passing into vapour 

 follow each other so very rapidly that the process 

 of melting is not usually observed. 



It is equally probable that in every motion what- 

 ever, even in the most gentle and cool that it is 



