THE HALOGENS 461 



about 100 In the laboratory the preparation ofchlorvne is carried on 

 in flasks, heated over a water-bath, by acting on manganese peroxide 



and hydrochloric acid does give chlorine at a red heat, and this reaction may also take 

 place at the moment of its evolution in this case. 



All the oxides of manganese (MnoO 3 , MnO 2 , MnO 3 , Mn 2 O 7 ), with the exception of man- 

 ganous oxide, MnO, disengage chlorine from hydrochloric acid, because manganous 

 chloride, MnCl 2 , is the only compound of chlorine and manganese which exists as a stable 

 compound, all the higher chlorides of manganese being unstable and evolving chlorine. 

 'Hence we here take note of two.-sepa&te"changes : (1) an exchange between oxygen and 

 chlorine, and (2) flie instability of theJjigher chlorine compounds. As (according to the 

 law of substitution) in the substitution of oxygeii by chlorine, C1 2 takes the place of O, 

 the chlorine compounds will contain more atoms than the corresponding oxygen 

 compounds. It is noli surprising, therefore, that certain of the chlorine compounds 

 corresponding "with oxygen compounds do not exist, or if they are formed are 

 very unstable". And furthermore, an atom of chlorine is_ heavier than an atom of oxygen, 

 and therefore a given element would have to retain a large mass of chlorine if in the 

 higher oxides the oxygen were replaced by chlorine. For this reason equivalent corn- 

 pounds of chlorine do not exist for all oxygen compounds. Many of the former are 

 immediately decomposed, when formed, with the evolution of chlorine. From this it is 

 evident that there should exist such chlorine compounds as would evolve chlorine as 

 peroxides evolve oxygen, and indeed a large "number of such compounds are known. 

 Amongst them may be mentioned antimony pentachloride, SbCl ? , which splits up into 

 chlorine "and antimony trichloride when heated. Cupric chloride, corresponding with 

 copper oxide, and having a composition CuCl 2 , similar to CuO, when heated parts with 

 half its chlorine, just as barium peroxide evolves half its oxygen. This method may 

 even be taken advantage of for the preparation of chlorine and -cuprous chloride, Cud. 

 The latter attracts oxygen from the atmosphere, and in so doing is converted from - 

 colourless substance into a green 'compound whose composition is Cu 2 Cl 2 O. With 

 hydrochloric acid this substance gives cupric chloride (Cu 2 Cl. 3 O + 2HCl = H 2 + 2CuCi 2 }, 

 which has only to be dried and heated jn order again to obtain chlorine. Thus, in solution, 

 and at the ordinary temperature, the compound CuCl 2 is stable, but when heated it- 

 splits up. On this property i's founded Deacon's process for the preparation of chlorine 

 from hydrochloric acid with the aid of air and copper salts, by passing a mixture of air and 

 hydrochloric acid at about 440 over bricks saturated with a solution of a copper salt 

 (a mixture of solutions "of CuS0 4 and NaoSO 4 ). CuCl 2 is then formed by the double 

 decomposition ot the salt of copper and the hydrochloric acid; the CuCl 2 liberates 

 chlorine, and the CuCl forms Cu 2 Cl 2 O with the oxygen of the air, which again gives CuCljr 

 with 2HC1, and so on. 



Magnesium chloride, which is obtained from sea-water, camallite, &c., may serve not* 

 only as a means for the preparation of hydrochloric acid, but also of chlorine, because 

 its basic salt (magnesium oxychloride) when heated in the air gives magnesium oxide and 

 chlorine (Weldon-Pechiney's process, 1888). Chlorine is now prepared on a large scale 

 by this method. Several new methods based upon this reaction have been proposed fo* 

 procuring chlorine from the bye-products of other chemical processes. Thus, Lyte and 

 Tattars (1891) obtained up to 67 p.c. of chlorine from CaCl 2 in this manner. A solu- 

 tion of CaCl 2 , containing a certain amount of common salt, is evaporated and oxide of 

 magnesium added to it. When the solution attains a density of 1-2445 (at 15), it is 

 treated with carbonic acid, which precipitates carbonate of calcium, while chloride of' 

 magnesium remains in solution. After adding ammonium chloride, the solution is 

 evaporated to dryness and the double chloride of magnesium and ammonium formed is 

 ignited, which drives off the chloride of ammonium. The chloride of magnesium which 

 remains behind is used in the Weldon-Pechiney process. The De Wilde-Eeychler (1892) 

 process for the manufacture of chlorine consists in passing alternate currents of hot aiv 

 and hydrochloric acid gas through a cylinder containing a mixture of the chlorides of 

 magnesium and manganese. A certain amount of sulphate of magnesium which does 



