M CHLORIDE BERTIK >LLKT S LAWS 449 



from the corresponding oxides in that the O is replaced by C1 2 , as 

 should follow from the law of substitution, because oxygen gives OH., 

 and is consequently bivalent, whilst chlorine forms HC1, and is there- 

 fore 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, because 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. The action of hydrochloric 

 acid, like that of every acid, on basic oxides, MO, and in general on 

 .M < > consists in a double decomposition, water, raH 2 O, and a metallic 

 chloride, M H C1 2JI ,, being formed. Hydrochloric acid, HC1, acts in the 

 same manner on the hydroxides of the bases M, 4 (OH) 2m -t- 2wHCJ 

 = '2mH.,O -t- M,,Cl 2m , and on carbonates, for instance, Na 2 CO 3 + 2HC1 

 = 2XaCl 4- H. 2 O + CO 2 . 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,,Cl. 2m without any oxygen bear the name of 

 haloid salts ; for instance, HC1 is a haloid acid, Nad a haloid salt, 

 chlorine a halogen. Certain higher degrees of combination of chlorine 

 with the metals disengage chlorine when heated to redness ; for in- 

 stance, the salt Cud., is converted into CuCl. The capacity of hydro- 

 chloric acid to give, by its action on bases, MO, a metallic chloride, 

 MCI.,, and water, is limited at high temperatures by the reverse re- 

 action MC1 2 -f H.,O = MO + 2HC1, and the more pronounced are 

 the basic properties of MO the feebler is the reverse reaction, while 

 for feebler bases such as A1 2 O 3 , MgO, <tc., this reverse reaction pro- 

 ceeds with ease. In this manner Deville obtained crystals of the 

 amorphous oxides Fe. 2 O 3 , SnO 2 , ttc., by passing a slow current of 

 hydrogen chloride over them at a red heat ; Fe 2 Cl G and SnCl 4 were 

 momentarily formed, but were again decomposed by the water liberated 

 and the liberated oxides were separated as crystals. Metallic chlorides 

 corresponding with the peroxides either do not exist, or are easily decom- 

 posed with the disengagement of chlorine. Thus there is no compound 

 BaCl 4 corresponding with the oxide 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 whilst CaCl 2 is easily fused), and many other salts. Under 

 the action of heat many are more stable than the oxides, many are even 

 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 

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