168 Mr. Colquhoun on the Life and W7itings [Mahch, 



the quantity of matter existing in each mass ; and as the law by 

 which the attractive force diminishes with the distance is accu- 

 rately known, they may, in every instance, be subjected to the 

 most rigorous calculation. 



Chemical affinity, on the contrary, consists in the reciprocal 

 attraction between the ultimate particles of substances, between 

 which there intervenes only an insensible distance. In these 

 circumstances, the mere attractive force, instead of acting undis- 

 turbed, is modified by the peculiar affections of the molecules, 

 as bv their figure, their distance, &c. ; and as these affections 

 undoubtedly vary in the ultimate particles of every different sub- 

 stance, it is obvious that the modifications which may thus be 

 produced upon the attractive force are infinite. And this is the 

 reason why it is impossible to estimate beforehand the amount 

 of attraction which will take place between the molecules of any 

 two substances : it must be investigated experimentally. 



If this doctrine be correct, the assumption made by Bergmann 

 and his predecessors, of an infinite number of distinct forces, all 

 varying in their intensity, was gratuitous, and altogether unne- 

 cessary : they are all the results of one great power, modified 

 more or less by the peculiar qualities of the substances in which 

 it acts. 



An example may serve to contrast in a still more striking 

 manner the opposite views respecting chemical combination to 

 which these two theories conduct. Suppose that to a mixture 

 of two acids we add a quantity of an alkali insufficient to neutral- 

 ize any one of them separately ; what will be the result? Accord- 

 ing to the theory of elective attraction, the alkali will attach 

 itself exclusively to the acid for which it possesses the most 

 powerful affinity ; while the whole of the other acid, and that 

 portion of the stronger acid in excess over what is necessary to 

 produce exact neutralization of the alkali, will remain in a disen- 

 gaged state in the liquid. Berthollet, on the contrary, main- 

 tained, that as the constant effect of attraction is combination, 

 and as the degree of attraction is proportional to the mass of the 

 attracting body, the two acids will share the alkali between 

 them, and the amount of alkali with which each will be com- 

 bined, will be in the compound proportion of its quantity and 

 the intensity of its attractive force. 



At first view, this theory may appear to leave unaccounted 

 for the decompositions which are such frequent consequences of 

 chemical action, and which are so admirably explained by Berg- 

 mann's principle of election. If, for example, to a solution of 

 acetate of lime we add oxalic acid, the whole of the lime will be 

 precipitated in the state of oxalate, and the supernatant liquid 

 will contain uncorabined acetic acid. If the constant effect of 

 affinity be combination, it may be asked, what is the reason that 

 in this instance the whole of the lime does not remain in solution, 



