456 PRINCIPLES OF CHEMISTRY 



consequently bivalent, whilst chlorine forms HC1, and is therefore 

 univalent. So, for instance, ferrous oxide, FeO, corresponds with 

 ferrous chloride, FeCl 2 , and the oxide Fe 2 O 3 with ferric chloride, which 

 is also seen from the origin of these compounds, for FeCl 2 is ob- 

 tained by the action of hydrochloric acid on ferrous oxide or- carbonate 

 and FeCl 3 by its action on ferric oxide. In a word, all the typical 

 properties of acids are shown by hydrochloric acid, and all the typical 

 properties of salts in the metallic chlorides .derived from it. Acids and 

 salts composed like HC1 and M n Cl 2m without any oxygen bear the name 

 of haloid salts ; for instance, HC1 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, MCI 2 , and water, is limited at ' 

 high temperatures by the reverse reaction MC1 2 + H 2 O = MO + 2HC1, 

 and the more pronounced are the basic properties of MO the feebler is 

 the reverse action, while for feebler bases such as A1 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 Ba01 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 corro- 

 sive 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 pre- 

 cipitates 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, 

 HgC 2 + Cu =s CuCl 2 + Hg, and hydrogen at a red heat displaces 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 reactions of double saline decompositions proceed in one or in the 

 other direction are determined by the properties o the compounds 

 which take part in the reaction, and of those capable of formation at the 



