oxygen bear the name of haloid salts; for instance, HCl is a haloid acid, NaCl a haloid salt, chlorine a halogen. The capacity of hydrochloric acid to give, by its action on bases, MO, a metallic chloride, MCl 2 , and water, is limited at high temperatures by the reverse reaction MCl 2 + H 2 O = MO + 2HCl, and the more pronounced are the basic properties of MO the feebler is the reverse action, while for feebler bases such as Al 2 O 3 , MgO, &c., this reverse reaction proceeds with ease. Metallic chlorides corresponding with the peroxides either do not exist, or are easily decomposed with the disengagement of chlorine. Thus there is no compound BaCl 4 corresponding with the peroxide BaO 2 . Metallic chlorides having the general aspect of salts, like their representative sodium chloride, are, as a rule, easily fusible, more so than the oxides (for instance, CaO is infusible at a furnace heat, whilst CaCl 2 is easily fused) and many other salts. Under the action of heat many chlorides are more stable than the oxides, some can even be converted into vapour; thus corrosive sublimate, HgCl 2 , is particularly volatile, whilst the oxide HgO decomposes at a red heat. Silver chloride, AgCl, is fusible and is decomposed with difficulty, whilst Ag 2 O is easily decomposed. The majority of the metallic chlorides are soluble in 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 + Cu = CuCl 2 + Hg, and hydrogen at a red heat displaces silver from silver chloride, 2AgCl + H 2 = Ag 2 + 2HCl. 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 reactions of double saline decompositions proceed in one or in the other direction are determined by the properties of the compounds which take part in the reaction, and of those capable of formation at the temperature, &c., as was shown in the preceding portions of this chapter, 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 SO3HCl; 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 composition 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, or as they give double salts. Compounds have long been known of hydrochloric acid with auric, platinic, and antimonious chlorides, and other similar metallic chlorides corresponding with very feeble bases. But Berthelot, Engel, and others have shown that the capacity of HCl for combining with M n Cl m is much more frequently encountered than was previously supposed. Thus, for