XVI INTRODUCTION. 



that of which it has been deprived. These aherations 

 of form and consistency do not affect the nature of 

 bodies ; but, by bringing into contact or separating the 

 molecules of which they are composed, they augment 

 or diminish their cohesion and their affinity, and thus 

 dispose them to form new combinations. 



The principles which I have just explained are not 

 rigorously applicable to animal and vegetable substances, 

 nor to some other compound bodies, except so far as 

 the effects of a low degree of heat are concerned. The 

 constituent principles of such bodies do not all require 

 the same degree of heat to cause them to pass to the 

 liquid or gaseous state. It follows, then, that some of 

 them can take the one or the other of these forms, by any 

 degree of heat above that of the atmosphere, and thus 

 be separated from those which remain fixed. In this 

 case decomposition is produced. 



If the force of affinity were the same amongst all the 

 elementary particles of which various bodies are com- 

 posed, there would be only confused aggregations of 

 matter, throughout all the operations of nature and art. 

 But each element has its peculiar affinities, which enable 

 it to enter, more or less closely, into combination with 

 certain other elements, whilst it strongly resists a union 

 with those for which it has no affinity. All matter is 

 formed, governed, and separated, according to these 

 different affinities. The uniform reproduction of the 

 combinations of art and the productions of nature is 

 derived from this principle. 



It follows from the preceding statement, that the force 

 of affinity alone can hold in lasting combination the par- 

 ticles of matter, — and that bodies are, even then, liable 



