xix BALANCED ACTIONS 505 



quantity to neutralise the whole of two bases, or conversely, 

 the competition is less keen, and the course of the action 

 is determined mainly by the order in which the salts are 

 precipitated from solution. In their aqueous solutions, 

 all neutral salts are almost equally strong or equally stable ; 

 moreover, unlike the neutralisation of an acid by an alkali, 

 no marked liberation of heat occurs when two soluble salts 

 are formed by " double decomposition " from two others 

 (Hess's "Law of Thermoneutrality," Ann. de Chim., 1842, 

 [iii], 4, 222). The nature of the salts that separate on 

 evaporating the solution is therefore determined more by 

 their relative solubilities than by any factor depending on 

 chemical affinity. 



Berthollet's experiments (1804) on the interaction of 

 neutral salts. In his Essay on Chemical Statics (1803; 

 tr. B. Lambert, 1804), Berthollet made a very careful study of 

 the balanced interactions of pairs of neutral salts. Amongst 

 other cases he studied the following : 



(a) One of the four salts is almost insoluble in water. In 

 this case it is precipitated almost completely when the 

 alternative pair of soluble salts is mixed. 



" If sulphate of potash and nitrate of lime are mixed, in 

 any proportions, the sulphate of lime which is formed 

 separates by the excess of its insolubility compared with 

 that of nitrate of potash " {Chemical Statics ', I. 70) : 



K 2 SO 4 + Ca(NO 8 ) 2 ^ CaSO 4 + 2KNO S . 



(precipitated) 



Barium sulphate, which is even less soluble than calcium 

 sulphate, is precipitated so completely that it can be filtered 

 off and weighed in order to estimate either the amount of 

 barium or the quantity of sulphates present in a solution : 



K 2 SO 4 + Ba(NO 3 ) 2 -> BaSO 4 + 2 KNO 3 . 



insoluble 



(b) Two salts are almost equally sparingly soluble. If 

 these two salts are on opposite sides of the equation, either 



