450 PRINCIPLES OF CHEMISTRY 



water, but silver chloride, cuprous chloride, mercurous chloride, and 

 lead chloride are sparingly soluble in water, and are therefore easily 

 obtained as precipitates when a solution of the salts of these metals is 

 mixed with a solution of any chloride or even with hydrochloric acid. 

 The metal contained in a haloid salt may often be replaced by another 

 metal, or even by hydrogen, just as is the case with a metal in an. 

 oxide. Thus copper displaces mercury from a solution of mercuric 

 chloride, HgCl 2 4- Cu = CuCl. 2 + Hg ; thus hydrogen at a red heat dis- 

 places silver from silver chloride, 2AgCl + H. 2 = Ag 2 + 2HC1. These, 

 and a whole series of similar reactions, form ^ the typical methods of 

 double saline decompositions. The measure of decomposition and the 

 conditions under which one or the other side of reactions of double 

 saline decompositions proceeds (for instance, whether a metal and an 

 acid give hydrogen and a salt or the reverse) are determined by the 

 properties of the compounds which are in action, and which are able 

 to be formed at the temperature, <fcc., as was shown when speaking of 

 sodium chloride, and as will be frequently found hereafter. 



If hydrochloric acid enters into double decomposition with basic 

 oxides and their hydrates, this is only due to its acid properties ; and 

 for the same reason it rarely enters into double decomposition with 

 acids and acid anhydrides. Sometimes, however, it combines with the 

 latter, as, for instance, with the anhydride of sulphuric acid, forming 

 the compound SO 3 HC1 ; and in other cases it acts on acids, giving up 

 its hydrogen to their oxygen and forming chlorine, as will be seen in 

 the following chapter. 



Hydrochloric acid, as may already be concluded from the compo- 

 sition of its molecule, belongs to the monobasic acids, and does not, 

 therefore, give true acid salts (like HNaSO 4 or HNaCO 3 ) ; nevertheless 

 many metallic chlorides, formed from powerful bases, are capable of 

 combining with hydrochloric acid, just as they combine with water, 

 or with ammonia, and as they give double salts. Compounds have long 

 been known of hydrochloric acid with auric, platinic, and antimo- 

 nious chlorides, and other similar metallic chlorides corresponding 

 with very feeble bases. But Berthelot, Engel, and others have shown 

 that the capacity of HC1 for combining with M n Cl m is much more 

 frequently encountered than was before supposed. Thus, for instance, 

 dry hydrochloric acid when passed into a solution of zinc chloride 

 (containing an excess of the salt) gives in the cold (0) a compound 

 HCl,ZnCL 2 ,2H 2 O,and at the ordinary temperature HCl,2ZnCl 2 ,2H 2 O, 

 just as it is able at low temperatures to form the cry stallo- hydrate 

 ZnCl 2 ,3H 2 O (Engel, 1886). Similar compounds are obtained with 

 CdCl 2 , CuCl 2 , HgCl 2 , Fe. 2 Cl 6J &c. (Berthelot, Ditte, Cheltzoff, Lachinoff, 



