FORMATION OF COMPOUNDS BY SUBSTITUTION. lyi 



in which the constituents of both salts before decomposition 

 inclosed in brackets, are found to have exchanged places after 

 decomposition, without any other change in the original salts.* 

 The double sulphate of lime and soda can be formed artificially 

 only in circumstances, which are somewhat similar. It is pro- 

 duced on adding sulphate of soda to acetate of lime, the sulphate 

 of lime, as it then precipitates, carrying down sulphate of soda 

 in the place of constitutional water (Liebig). 



Different hydrates of the same body such as peroxide of tin, 

 differ sensibly in properties, and afford different compounds 

 with acids, unquestionably because these compounds are formed 

 by substitution. The constant formation of phosphates con- 

 taining one, two or three atoms of base, on neutralizing the cor- 

 responding hydrates of phosphoric acid with a fixed base, like- 

 wise illustrates in a striking manner the derivation of compounds, 

 on this principle. Many insoluble substances, such as the earth 

 silica, possess a larger proportion of water, when newly precipi- 

 tated, than they retain afterwards, and in that high state of 

 hydration they may exhibit affinities for certain bodies which 

 do not appear in other circumstances. Hydrated silica dissolves 

 in water at the moment of its separation from a caustic alkali ; 

 and alumina dissolves readily in ammonia, when produced in 

 contact with that substance by the oxidation of aluminum. 

 The unusual disposition to enter into combination which silica 

 and alumina then exhibit is generally ascribed to their being in 

 the nascent state, a body at the moment of its formation and 

 liberation, in consequence of a decomposition, being, it is sup- 

 posed in a favourable condition to enter anew into combination. 

 But their degree of hydration in the nascent state may be the 

 real cause of their superior aptitude to combine. 



Double decompositions take place without the great evolution 

 of heat, which often accompanies the direct combination of two 

 bodies, and with an apparent facility or absence of effort, as if 

 the combinations were just balanced by the decompositions, 

 which occur at the same time. It is perhaps from this cause 

 that the result of double decomposition is so much affected by 

 circumstances, particularly by the insolubility of one of the 

 compounds. For it is a general law to which there is no excep- 

 tion, that two soluble salts cannot be mixed without the occur- 



* On water as a constituent of sulphates, Phil. Mag. 3d series, vol. VI. p. 417. 



