190 CHEMICAL AFFINITY. 



It is remarkable that compounds are in general more easily 

 formed by substitution, than by the direct union of their con- 

 stituents ; indeed many compounds can be formed only in that 

 manner. Carbonic acid is not absorbed by anhydrous lime, 

 but readily by the hydrate of lime, the water of which is dis- 

 placed in the formation of the carbonate. In the same manner, 

 ether, although a strong base does not combine directly with 

 acids, but the salts of ether are derived from its hydrate or 

 alcohol, by the substitution of an acid for the water of the 

 alcohol. In all the cases, likewise in which hydrogen is evolved 

 during the solution of a metal in a hydrated acid, a simple sub- 

 stitution of the metal for hydrogen occurs. 



Combination takes place with the greatest facility of all 

 when double decomposition can occur. Thus carbonate of lime 

 is instantly formed and precipitated, when carbonate of soda is 

 added to nitrate of lime, nitrate of soda being formed at the same 

 time and remaining in solution. 



Before decomposition. After decomposition. 



Carbonate of f Soda. . . ^ Nitrate of soda 



soda. I Carbonic acid x.x^ 



Nitrate of / Nitric acid. '^*^x^ 



lime I Lime. . ^.Carbonate of lime. 



Here a doVble substitution occurs, lime being substituted for 

 soda in the carbonate, and soda for lime in the nitrate. Such 

 reactions may therefore be truly described as double substitu- 

 tions as well as double decompositions. They are most com- 

 monly observed on mixing two binary compounds or two salts. 

 But reactions of the same nature may occur between compounds 

 of a higher order, such as double salts, and new compounds be 

 thus produced, which cannot be formed by the direct union of 

 their constituents. Thus the two salts, sulphate of zinc and 

 sulphate of soda, when simply dissolved together, always cry- 

 stallize apart, and do not combine. But the double sulphate of 

 zinc and soda is formed on mixing strong solutions of sulphate 

 of zinc and bisulphate of soda, and separates by crystallization ; 

 the sulphate of water with constitutional water (hydrated acid of 

 sp. gr. 1.78) being produced at the same time and remaining in 

 solution. The reaction which occurs may be thus expressed : 



Before decomposition. After decomposition. 



HO, S0 3 + (NaO, S0 3 ) If HO, SO 3 + HO 

 ZnO, S0 3 + (HO) / " I ZnO, SO 3 .+ NaO, SO 3 



