(>XV(iKN AM> ITS SALINE COMBINATIONS 189 



acid oxides, or the result of the action of hydrates of these cl;i 

 on each other, with separation of water. But salts may be obtained 

 by other methods. Let us not forget that basic oxides are formed 

 by metals, and acid oxides often by non-metals. But metals and 

 non-metals are capable of combining together, and a salt is frequently 

 formed by the oxidation of such a compound. For example, iron very 

 easily combines with sulphur, forming iron sulphide (as we saw in the 

 Introduction) ; this in air, and especially moist air, absorbs oxygen, 

 with the formation of the same salt as may be obtained by the combina- 

 tion of the oxides of iron and sulphur, or of the hydrates of these 

 oxides. Hence, it cannot be said or supposed that a salt contains 

 the principles of the oxides, or that a salt must necessarily contain two 

 kinds of oxides in itself. The same conclusion may be arrived at by 

 investigating the different other methods of the formation of salts 

 thus, for instance, many salts enter into double decomposition with the 

 metals, in which case the acting metal replaces that which originally 

 occurred in the salt. As we saw in the Introduction, iron, when placed 

 in a solution of copper sulphate, separates out the copper, and forms 

 an iron salt. Thus, the derivation of salts from oxides, is only 

 one of the methods of their preparation, there being many others, 

 and, therefore, it cannot be affirmed that a salt is simply the compound 

 of two oxides. We saw, for instance, that in sulphuric acid it was 

 possible to replace the hydrogen by zinc, and that by this means zinc 

 sulphate was formed ; so likewise the hydrogen in many other acids 

 may be replaced by zinc, iron, potassium, sodium, and a whole series of 

 similar metals, corresponding salts being obtained. The hydrogen in 

 the water of the acid, in this case, is exchanged for a metal, and a salt 

 is obtained from the hydrate. In this sense of a salt it may be said, 

 that a salt is an acid in which hydrogen is replaced by a metal. Such 

 a definition will be much more exact than that previously given, for it 

 refers directly to elements and not to their compounds with oxygen. 

 It shows that a salt and an acid are essentially compounds of the same 

 series, with the difference that the latter contains hydrogen and the 

 former a metal. Such a definition is still more exact than the first 

 definition of salts in respect to its referring likewise to those acids 

 which do not contain oxygen, and, as we shall afterwards learn, there 

 is a series of such acids. Such elements as chlorine and bromine form 



ciating, evolves carbonic anhydride. The same gas, when dissolved in solutions of salts, 

 acts in one or the other manner (see Chap. II., Note 88). Here it is seen what a successive 

 series of relations exists between compounds of a different order, between sub- 

 stances of different degrees of stability. Were solutions distinctly separated from 

 chemical compounds, we should not be able to see those natural transitions which exist 

 in reality. 



