CONSTITUTION OF SALTS. 171 



means indiscriminately. With a few exceptions which may be 

 placed out of consideration for the present, the combining salts 

 have always the same acid, sulphates combining with sulphates, 

 chlorides with chlorides. Their bases or their metals, however, 

 must belong to different natural families. Thus it may be 

 questioned whether a salt of potash ever combines with a salt 

 of soda, certainly never with a salt of ammonia. Salts of the 

 numerous metals including hydrogen, belonging to the magne- 

 sian family, do not combine together ; thus sulphate of magne- 

 sia does not form a double salt with sulphate of lime, with 

 sulphate of zinc or with sulphate of water. While on the other 

 hand salts of this family are much disposed to combine with 

 salts of the potassium family ; sulphate of soda, for instance, 

 forming double salts with sulphate of lime, sulphate of zinc 

 and sulphate of water. We have thus the means of distinguish- 

 ing between a double salt, and the salt of a bibasic or tribasic 

 acid. The bisulphate and binoxalate of potash, saturated with 

 soda, form sulphates and oxalates of potash and soda, which 

 separate from each other by crystallization, although the acid 

 salts are themselves double salts of water and potash. But the 

 acid fulminate of silver, or the acid tartrate of potash (bitar- 

 trate) affords only one salt when saturated with soda, in which 

 isomorphous bases exist, and which, therefore, is a salt of one 

 acid, and not a compound of two salts. The great proportion 

 of the salts which are named super, acid and #i-salts, contain 

 a salt of water and are double salts, such as the supercarbo- 

 nateof soda (HO, CO 2 + NaO, CO 2 ), the acid sulphates of 

 potash and the binacetate of soda; but a few of them are bibasic or 

 tribasic salts, containing one or two atoms of water as base, 

 such as the salt called bitartrate of potash, and biphosphate 

 of potash (2HO, KO+PO 5 j. 



There is no parallelism between the constitution of a double 

 salt, and that of a simple salt itself, or foundation for the state- 

 ments which are sometimes made, that one of the salts which 

 compose a double salt has the relation to the other of an acid 

 to a base, and that one salt is electro-negative to the other. 

 The resolution of a double salt into its constituent salts by 

 electricity, has never been exhibited, and is not to be expected 

 from what is known of electrolytic action. While no analogy 

 whatever subsists between a double salt and a simple salt on 

 the binary view of the constitution of the latter. Besides, the 



