TIIK HALodKNs 455 



chloric acid requires a temperature of about 100. In the laboratory 

 the preparation <>f r///(>,-hte is carried on in flasks, heated over a water 

 bath, by acting on manganese peroxide with hydrochloric acid or a 



manganese peroxide, MnO >, breaks up into manganous oxide, MiiO, and oxygen, both of 

 which react with hydrochloric acid, the manganous oxide acting as abase, MnO + 2HC1 = 

 MnCl , H ._,(), and at the same time '2HC1 + O = ELO + CL. In reality a mixture of 

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

 place at the moment of its evolution in this case. 



All the oxides of manganese (Mn.,O-, MnO.,, MnO-,, Mn-jOy), with the exception of man- 

 ganou s ox ide. MnO, disengage chlorine from hydrochloric acid, because manganous chloride, 

 MnCL, 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. The reaction 

 between hydrochloric acid and salts containing much oxygen is explained according to the 

 law of substitution by the double decomposition and instability of the higher chlorides; 

 thus, for example, by taking potassium dichromate, K,Cr.>O 7 , a compound, K,Civ>Cl|.,, 

 might be formed, but it is unknown, and if it is formed it in all probability immediately 

 decomposes into chlorine, (5C1, potassium chloride, 2KC1, and chromium chloride, Cr.^Clg. 

 Hence we here encounter two circumstances : (1) a substitution between oxygen and chlo- 

 rine, and (2) the instability of those higher chlorine compounds. Both these circum- 

 stances have a very important signification for the comprehension of the relation of 

 such elements as chlorine and oxygen. As (according to the law of substitution) in the 

 substitution of oxygen by chlorine, CL takes the place of O, therefore the chlorine com- 

 pounds will contain in themselves more atoms than the corresponding oxygen compounds. 

 It is not surprising, therefore, that certain of the chlorine compounds corresponding with 

 oxygen compounds do not exist, or if they are formed are very unstable. And further- 

 more, 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 compounds of chlorine do not 

 exist for all oxygen compounds. Many of the former are immediately decomposed, when 

 formed, with the evolution of chlorine. Therefore chlorine is evolved by the action of 

 hydrochloric acid on compounds which contain much oxygen. 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, SbCls, which splits up into chlorine and antimony 

 trichloride when heated. Cupric chloride, corresponding with copper oxide, and having 

 a composition CuClo, similar to CuO, when heated parts with half its chlorine, just a- 

 barium peroxide evolves half its oxygen. This method may even be taken advantages of 

 for the preparation of chlorine and cuprous chloride, CuCl. The latter attracts oxygen from 

 the atmosphere, and in so doing is converted from a colourless substance into a green 

 compound whose composition is CuoCLO. With hydrochloric acid this substance gives 

 cupric chloride (CuoCloO + 2HC1 = H.^O + 2CuCl 3 ), which has only to be dried and heated, 

 and it again evolves chlorine. Thus, in solution, and at the ordinary temperature, the 

 compound CuCl., it, ((instant, but when heated it splits up. On this property is founded 

 Deacon's process for the preparation of chlorine from hydrochloric acid by the aid of air 

 and copper salts, by passing a mixture of air and hydrochloric acid at about 440 over 

 bricks soaked with a solution of a copper salt (a mixture of solutions of CuSO 4 and Na^SOj). 

 CuClo is then formed by the double decomposition of the salt of copper and the hydro- 

 chloric acid ; the CuClo liberates chlorine, and the CuCl forms CuoCLO with the oxygen of 

 the air, which again gives CuClo with 2HC1, and so on. 



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

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

 (Weldon-Pechiuv's process) its basic salt (magnesium oxychloride) when heated in the 

 air gives magnesium oxide and chlorine. Chlorine is now prepared on a large scale by 

 this method. 



