38 HISTORY OF CHEMISTRY. [LECTURE III. 



sequence of the two opposed forces, cohesion and elasticity. 

 The predominance of the former brings about the solid state, 

 and that of the latter, the gaseous state; while in liquids a 

 balance exists between the two. If all acids were in the same 

 state of condensation, then that one should be regarded as the 

 strongest, of which the smallest quantity is necessary to saturate 

 a given weight of a base ; or, as we should now say, of which 

 the equivalent is smallest. 



Berthollet applies these principles especially to simple and 

 to double decompositions. According to him, when we add an 

 acid to a dissolved salt, a partition of the base takes place 

 between the two acids in the ratio of their affinities, that is, in 

 the ratio of their masse chtmiytje^ Both salts and both acids 

 are thus present in the solution ; yet this only holds when both 

 salts possess approximately the same solubility, because a 

 balance is then established which is dependent not only upon 

 the strengths of the acids, but also, and in particular, upon the 

 quantities of each present. He further draws attention to the 

 fact that conviction as to the accuracy of this view cannot be 

 obtained by evaporating the solution and permitting the salts 

 to crystallise out, because as soon as the quantity of water is 

 no longer sufficient for complete solution, the phenomena which 

 take place depend chiefly upon the forces of cohesion and 

 crystallisation, that is, upon the different solubilities of the 

 substances. 16 



Thus, upon crystallising out after mixing a solution of 

 potassium nitrate with sulphuric acid, the more sparingly 

 soluble salt potassium sulphate is alone obtained ; whereas, 

 according to Berthollet, potassium nitrate and potassium sul- 

 phate are both present in the solution. 



When one salt is much more soluble than the other, it is 

 the less soluble one that is principally formed ; and if one is 

 quite insoluble, there is no partition, but complete decomposi- 

 tion. In this way Berthollet explains, for example, the com- 

 plete precipitation of barium nitrate by means of sulphuric 



15 Stat. Chim. I, 75 ; E, I, 49. 16 Ibid. I, 82 ; E. I, 54, 



