VII. CHEMICAL AFFINITY. 



CHEMICAL AFFINITY, or the force by .which dissimi- 

 \_J lar bodies tend to unite and form compounds differing 

 generally in character from their constituents, is that mode of 

 force of which the human mind has hitherto formed the least 

 definite idea. The word itself affinity is ill chosen, its 

 meaning, in this instance, bearing no analogy to its ordinary 

 sense ; and the mode of its action is conveyed by certain con 

 ventional expressions, no dynamic theory of it worthy of 

 attention having been adopted. Its action so modifies and 

 alters the character of matter, that the changes it in 

 duces have acquired, not perhaps very logically, a generic 

 contradistinction from other material changes, and we 

 thus use, as contradistinguished, the terms physical and 

 chemical. 



The main distinction between chemical affinity and physi 

 cal attraction or aggregation, is the difference of character of 

 the chemical compound from its components. This is, how 

 ever, but a vague line of demarcation ; in many cases, which 

 would be classed by all as chemical actions, the change of 

 character is but slight ; in others, as in the effects of neutrali 

 sation, the difference of character would be a result which 

 would equally follow from physical attraction of dissimilar 

 substances, the previous characters of the constituents depend 

 ing upon this very attraction or affinity : thus an acid corrodes 



