448 PRINCIPLES OF CHEMISTRY 



ash, lime, <kc. In a dry state, however, hydrochloric acid does not alter 

 vegetable dyes, and does not accomplish 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, &c., 

 act on gaseous hydrochloric acid, displacing the hydrogen and leaving 

 half a volume of hydrogen against one volume of hydrochloric acid gas ; 

 this reaction may serve for determining the composition of hydrochloric 

 acid. Combined with water the hydrochloric acid gas is deprived of its 

 elasticity, and loses a considerable amount of heat in its passage into 

 a liquid and combined state. It then acts as an acid which much 

 resembles nitric acid 42 in its energy and in many of its reactions ; how- 

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

 and so very frequently acts as an oxidiser, which hydrochloric 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 SO 2 ) displace the hydrogen from hydrochloric acid. Thus 

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

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

 platinum. Lead is only acted on feebly in compact masses, 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 respect 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 hydrox- 

 ides, 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 , 

 Fe 2 Cl 6 , PtCl 4 , &c. Many metals, as will have been remarked in the 

 preceding examples, give several degrees of combination with chlorine, 

 as with hydrogen. In their composition the metallic chlorides differ 



42 Thus, for instance, with feeble bases they evolve in dilute solutions (Chapter III. 

 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- 

 stant boiling point ; in the solutions of both (with HCl,6HoO and with NHO 5 ,5H 2 O) th- 



direction of the differential da/ dp varies, &c. 



