19<2 CHEMICAL AFFINITY. 



rence of decomposition, if one of the products that may be 

 formed is as insoluble salt. On mixing carbonate of soda and 

 nitrate of lime, the decomposition seems to be determined 

 entirely by the insolubility of the carbonate of lime, which 

 precipitates. When sulphate of soda and nitrate of potash are 

 mixed, no visible change occurs, and it is doubtful whether the 

 salts act upon each other, but if the mixed solution be concen- 

 trated, decomposition occurs and sulphate of potash separates 

 by crystallization owing to its inferior solubility. 



It may sometimes be proved that double decomposition occurs 

 on mixing soluble salts, although no precipitation supervenes. 

 Thus on mixing strong solutions of sulphate of copper and 

 chloride of sodium, the colour of the solution changes from 

 blue to green, which indicates the formation of chloride of 

 copper, and consequently that of sulphate of soda also. Now 

 it is known that hydrochloric acid will displace sulphuric acid 

 from the sulphate of copper, at the temperature of the experi- 

 ment, while sulphuric acid will on the other hand displace 

 hydrochloric from chloride of sodium. It hence appears that 

 in the preceding double decomposition, those acids and bases 

 unite which have the strongest affinity for each other, and the 

 same thing may happen on mixing other salts. But where the 

 order of the affinities for each other of the acids and bases is 

 unknown, the occurrence of any change upon mixing salts, or 

 the extent to which the change proceeds, is entirely matter of 

 conjecture. 



It was the opinion of Berthollet, founded principally upon the 

 phenomena of the double decompositions of salts, that decom- 

 positions are at all times dependent upon accidental circum- 

 stances, such as the volatility or insolubility of the product, and 

 never result from the prevalence of certain affinities over others; 

 and consequently that in accounting for such changes, the con- 

 sideration of affinity may be neglected. He supposed that when 

 a portion of base is presented at once to two acids, it is divided 

 equally between them, or in the proportion of the quantities of 

 the two acids, and that one acid can come to possess the base 

 exclusively, only when it forms a volatile or an insoluble com- 

 pound with that body, and thereby withdraws it from the solution 

 and from the influence of the other acid. His doctrine will be 

 most easily explained by applying it to a particular case, and 

 expressing it in the language of the atomic theory. The reaction 



