THE HALOGENS 46.7 



platinum, 17 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 em- 

 ployed 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 hydro- 

 chloric 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 act* 

 ing on hydrochloric acid evolves chlorine. If the chlorine evolved be 

 transferred to a metal, then a fresh quantity is formed from the 

 remaining acids and also combines with the metaL 18 Thus the aqua 

 regia acts by virtue of the chlorine which it contains and disengages. 



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

 sulphur and phosphorus burn in it and" combine with it at the ordinary 

 temperature. Only nitrogen, carbon, and oxygen do not combine 

 directly with it. The chlorine compounds formed by the non^met.als 

 for instance, phosphorus trichloride, PC1 3 , and sulphurous chloride, 

 &c., do not have the properties of salts, and, as we shall afterwards see 

 more fully, correspond to acid anhydrides and acids ; for example, PC1 3 

 to phosphorous acid, P(OH) 3 



NaCl FeCl 2 SnCl 4 PC1 3 HOI 

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



gas-conducting tube furnished with a glass cock is placed in the cork closing it, and the 

 air is pumped out of the globe. The gas-conducting tube 'is then connected with a vessel 

 containing chlorine, and the cock opened ; the chlorine rushes in, and the metallic leaves 

 are consumed. 



17 The behaviour of platinum to chlorine at a high temperature (1,400) is very 

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

 composes at a much ,lower^temperature into chlorine and platinum. Hence, 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). However, V. Meyer was- able to observe the 

 density of chlorine in a platinum vessel at 1,690, at which temperature chlorine does not 

 exert this action 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 acuon of aqua regia, showed that 

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

 acid, NO 2 C1 (nitric acid, N0 2 OH, in which HO is replaced by chlorine ; see Chapter oft 

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

 formation of aqua regia may therefore be expressed by 4NH0 5 +8HC1=2NO 2 C1+2NOC1 

 + 6H<jO + 2C1 2 . The formation of the chlorides NO 2 C1 and NOC1 is explained by the fact 

 that the nitric acid is deoxidised, gives the oxides NO and N0 2 , and they directly combine 

 with chlorine to form the above anhydrides. 



