THK HALOGENS 461 



which do not combine directly with oxygen and give very unstable 

 compounds with it, unite directly with chlorine to form metallic- 

 chlorides. Either chlorine water or aqua regia may be employed for 

 this purpose instead of gaseous chlorine. These dissolve gold and 

 platinum, converting them into metallic chlorides. Aqua regia is a 

 mixture of 1 part of nitric acid with 2 to 3 parts of hydrochloric acid. 

 This mixture converts into soluble chlorides not only those metals 

 which are acted on by hydrochloric and nitric acids, but also gold and 

 platinum, which are insoluble in either acid separately. This action 

 of aqua regia depends on the fact that nitric acid in acting 011 hydro- 

 chloric acid deprives it of its hydrogen, and evolves chlorine little by 

 little as the action proceeds. If the chlorine evolved be transferred to- 

 a metal, then a fresh quantity is formed from the remaining acids and 

 combines with the metal. 18 Thus the aqua regia acts in virtue of the 

 chlorine which it contains and disengages. 



The majority of non-metals also react directly on chlorine ; sulphur 

 and phosphorus burn in it, and combine with it directly when heated 

 or even at the ordinary temperature. Only nitrogen, carbon, and oxygen 

 do not form any direct compounds with it. The chlorine compounds, 

 formed by the non-metals for instance, phosphorus trichloride, PC1 3 , 

 and sulphurous chloride, Arc., do not have the properties of salts, and 

 if the metallic chlorides M,,Cl. 2m correspond with bases M }i O w and their 

 hydrates M,,(OH)., m , then, as we shall afterwards see more fully, the- 

 chlorides of the non-metals bear the same relation to acid anhydrides- 

 and acids : 



NaCl FeCl 2 SnCl 4 PC1 3 HC1 

 Na(HO) Fe(HO) 2 Sn(HO) 4 P(HO) 3 H(HO) 



remarkable, because platinous chloride, PtCL, is then formed, whilst this substance de- 

 composes at a much lower temperature into chlorine and platinum. Therefore, when 

 chlorine comes into contact with platinum at such high temperatures it forms fumes of 

 platinous chloride, and they on cooling decompose with the liberation of platinum, so 

 that the phenomenon appears to be dependent on the volatility of platinum. Deville 

 proved the formation of platinous chloride by inserting a cold tube inside a red-hot one 

 (as in the experiment on carbonic oxide, p. 388). However, Meyer was able to observe- 

 the density of chlorine in a platinum vessel at 1(590, at which temperature chlorine does 

 not exert this acjtion on platinum, or at least only to an insignificant degree. 



18 When left exposed to the air aqua regia disengages chlorine, and afterwards it no 

 longer acts on gold. Gay-Lussac, in explaining the action of aqua regia, showed that when 

 heated it evolves, besides chlorine, the vapours of two chloranhydrides that of nitric 

 acid, NOoCl (nitric acid, NOoOH, in which HO is replaced by chlorine, see chapter on 

 Phosphorus), and that of nitrous acid, NOC1 (ibid.} but these do not act on gold. The 

 formation of aqua regia may therefore be expressed by 4NHO 3 + 8HC1 = 2NO 2 C1 -I- 2NOC1 + 

 tfH 2 O + 2C1. 2 . The formation of the chlorides NO 2 C1 and NOC1 is explained by the fact 

 that the nitric arid is deoxidised, gives the oxides NO and NO..,, and they directly combine 

 with chlorine to form the above anhydrides. 



