374 ' Of the Propagation of Heat 



are found to exist, namely, that of a solid, that of a 

 fluid, and that of an elastic fluid, or gas ; and it is well 

 known that every substance with which we are acquainted 



all ponderable matter without exception is capable 

 of existing alternately under all those forms indifferent- 

 ly ; and that the form under which it appears at any given 

 time depends on its temperature at that time. 



We know farther that every identical substance under- 

 goes these different changes of form at certain fixed tem- 

 peratures ; and when we consider the subject with atten- 

 tion we shall find that, had not these temperatures been 

 fixed, and had they not been different in different bodies, 

 it would have been utterly impossible for us to have iden- 

 tified any substance whatever. 



Perhaps this is the only essential difference that really 

 exjsts among bodies that appear to us to be different. 



But not only the degrees of Heat, or points in the 

 scale of temperature at which the forms of different 

 bodies are changed, are various, but the extent of the va- 

 riation of temperature under which a substance can perse- 

 vere, or continue to maintain its form in its middle state, 



that of fluidity, or rather liquidity, is very different in 

 different bodies ; and this last circumstance has a won- 

 derful effect in increasing the variety of the compositions 

 and decompositions which are continually taking place 

 in the various operations of nature on the surface of the 

 globe. 



Another circumstance, not less prolific in events, is the 

 union which takes place between bodies of different kinds ; 

 and those most important changes in regard to the de- 

 grees of Heat which the bodies so united can support 

 without having their forms changed, which are found to 

 result from such union. 



