SODIUM CHLORIDE BERTHOLLET'S LAWS 455 



vegetable dyes, and does not effect many double decompositions which 

 easily take place in the presence of water. This is explained by the 

 fact that the gaso-elastic state of the hydrochloric acid prevents its 

 entering into reaction. However, incandescent iron, zinc, sodium, <fec., 

 act on gaseous hydrochloric acid, displacing the hydrogen and leaving 

 half a volume of hydrogen for each volume of hydrochloric acid gas j 

 this reaction may serve for determining the composition of hydrochloric 

 acid. Combined with water hydrochloric acid acts as an acid 

 much resembling nitric acid 42 in its energy and in many of its reactions ; 

 however, the latter contains oxygen, which is disengaged with great ease, 

 and so very frequently acts as an oxidiser, which hydrochloric acid is not 

 capable of doing. The majority of metals (even those which do not 

 displace the H from H 2 SO 4 , but which, like copper, decompose it to the 

 limit of S0 2 ) displace the hydrogen from hydrochloric acid Thus 

 hydrogen is disengaged by the action of zinc, and even of copper and 

 tin. 42bi8 Only a few metals withstand its action ; for example, gold 

 and platinum. Lead in compact masses is only acted on feebly, 

 because the lead chloride formed is insoluble and prevents the further 

 action of the acid on the metal. The same is to be remarked with re- 

 spect to the feeble action of hydrochloric acid on mercury and silver, 

 because the compounds of these metals, AgCl and HgCl, are insoluble 

 in water. Metallic chlorides are not only formed by the action of 

 hydrochloric acid on the metals, but also by many other methods ; for 

 instance, by the action of hydrochloric acid on the carbonates, oxides, 

 and hydroxides, and also by the action of chlorine on metals and certain 

 of their compounds. Metallic chlorides have a composition MCI ; for 

 example, NaCl, KC1, AgCl, HgCl, if the metal replaces hydrogen 

 equivalent for equivalent, or, as it is Said, if it be monatomic or 

 univalent. In the case of bivalent metals, they have a composition 

 MC1 2 ; for example, CaCl 2 , CuCl 2 , PbCl 2 , HgCl 2 , FeCl 2 , MnCl 2 . The 

 composition of the haloid salts of other metals presents a further 

 ^variation ; for example, A1C1 3 , PtCl 4 , &c. Many metals, for instance 

 Fe, give several degrees of combination with chlorine (FeCl 2 ,FeCl 3 ) 

 P4S with hydrogen. In their composition the metallic chlorides differ 

 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 2 , and is 



' Thus, for instance, with feeble bases they evolve in dilute solutions (Chapter IIL, 

 Note 58) almost equal amounts of heat ; their relation to sulphuric acid is quite identical. 

 They both form fuming solutions as well as hydrates ; they both form solutions of con* 

 etant boiling point. 



48 bis Pybalkin (1891) found that copper begins to disengage hydrogen at 100, and 

 that chloride of copper begins to give up its chlorine to hydrogen gas at 280 ; for silver 

 these temperatures are 117 and 260 that is, there is less difference between them. 



