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Library. 



CHEMICAL AFFINITY. 



CHEMICAL AFFINITY, or the force by which dissimilar 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 conventional expres- 

 sions, 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 induces 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 

 be expected to 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 

 because it tends to unite with another body ; when united, its 

 corrosive power, i.e. its tendency to unite, being satiated, it 

 cannot, so to speak, be further attracted, and it necessarily 

 loses its corrosive power. But there are other cases where no 

 such result could a priori be anticipated, as where the attrac- 

 tion or combining tendency of the compound is higher than 

 that of any of its constituents ; thus, who could, by physical 



