Equilibrium in Salt- Solutions. 151 



(3) Partial double decomposition, 



become 



0-s)MR + (y-s)M'R' + *M'R + zMRf, 

 where 



x-=-y >z. 



Cases (1) and (2) are essentially the same, being comple- 

 mentary to one another. Case (3) is usually referred to as 

 the Law of Berthollet ; and though long regarded as a correct 

 statement of the facts of the case, has been recently shown by 

 Berthelot to be untrue in certain instances ; the general con- 

 clusion to be drawn from his work being that, when a salt of 

 a weak acid and a strong base is mixed with a salt of a strong- 

 acid and a weak base in solution, then Case (2) holds good, 

 complete double decomposition ensuing ; the cause w^hich 

 determines the course of the reaction being the superior affi- 

 nity of the strong base and the strong acid for one another, 

 which together with that of the weak acid for the weak base 

 produces a development of heat greater than that required for 

 the decomposition of the original salts. This is in strict 

 agreement with the. principle of maximum work. Berthelot's 

 experiments * in support of the above comprised salts of 

 (NH 4 ), Na, K, Fe,Zn, &c. with CI, (C 2 H 3 2 ), (N0 3 ), (S0 4 ), 

 (C0 3 ), &c, and are numerous enough to guarantee the general 

 truth of his conclusions. With regard, however, to the state 

 of equilibrium existing between two strong acids and two 

 strong bases, he readily enough admits the inefficiency of the 

 thermal-change method of investigation to solve the problem, 

 but inclines to a belief in the general truth of Berthollet's 

 law, and believes it probable that all four salts exist ready 

 formed in the solution. This conclusion is based partially on 

 the fact that when a concentrated solution, of two salts of dif- 

 ferent acids and bases, is exposed to varying temperatures, it 

 may deposit now one salt and now another. 



" Cette separation, due a la moindre solubilite du sel qui 

 se separe, implique cependant F existence simultanee des deux 

 sels .... dans la liqueur; en effet si Pun des sels ne preexistait 

 pas,aucune proportion de ce corps ne pourrait jamais se separer 

 sous forme de cristeaux"f. 



In my former paper I gave the results of experiments on 

 NaCl + KN0 3 and KCl + NaN0 3 , and stated my opinion 

 that in a solution of either pair of salts the former couple 

 NaCl + KN0 3 is always formed, to the probable total exclusion 



* Mecanique Chimique, vol* ii. f^Ibid. p. 716. 



