194 CHEMICAL AFFINITY. 



Here the affinity of the sulphuric and nitric acids, for potash 

 is supposed to be equal ; and the complete decomposition of 

 the nitrate of potash by the former acid which takes place, is 

 ascribed to the volatility of the latter acid, which, by occa- 

 sioning its removal in proportion as it is liberated, causes the 

 fixed sulphuric acid to be ever in excess. 



Complete decompositions in which the precipitation of an 

 insoluble substance occurs, were explained by Berthollet in 

 the same manner. On adding a portion of barytes to sulphate 

 of soda, the barytes decomposes the salt, and acquires sul- 

 phuric acid, till that acid is divided between the two bases in 

 the proportion in which they are present, and, at this point, 

 decomposition would cease, were it not that the whole sulphate 

 of barytes formed, is removed by precipitation. But a new 

 formation of that salt is the necessary consequence of that 

 equable partition of the acid between the two bases in contact 

 with it, which is the condition of equilibrium; and the new 

 product precipitating, more and more of it is formed, till the 

 sulphate of soda is entirely decomposed, and its sulphuric acid 

 removed by an equivalent of barytes. 



According to these views of Berthollet, no decomposition 

 should be complete, unless the product be volatile or insoluble, 

 as in the cases instanced. But such a conclusion is not con- 

 sistent with observation, as it can be shewn that a body may 

 be separated completely from a compound, and supplanted by 

 another body, although none of the products is removed by 

 the operation of either of the causes specified, but all continue 

 in solution and in contact with each other. Thus the salt borax, 

 which is a borate of soda, is entirely decomposed by the addi- 

 tion to its solution of a quantity of sulphuric acid, not more than 

 equivalent to its soda, although the liberated boracic acid 

 remains in solution ; for the liquid imparts to blue litmus 

 paper a purple or wine-red tint, which indicates free boracic 

 acid, and not that characteristic red tint, resembling the red of 

 the skin of the onion, which would inevitably be produce^d 

 by the most minute quantity of the stronger acid, if free. But 

 if the borax were only decomposed in part in these circum- 

 stances, and its soda equally divided between the two acids, 

 then free sulphuric, as well as boracic acid, should be found 

 in the solution. The complete decomposition of the salt can 



