456 PRINCIPLES OF CHEMISTRY 



mixture of common salt and sulphuric acid 4 and washing the gas with 

 water to remove hydrochloric acid. The action of hydrochloric acid on 

 bleaching powder (p. 161) gives chlorine without the aid of heat : 

 CaCl 2 O 2 + 4HCl=CaCl, + 2H,O + 2Cl. 2 , and is therefore also taken 

 advantage of in the laboratory for the preparation of chlorine/' 

 Chlorine cannot be collected over mercury, because it combines with it 

 as with many other metals, and it is soluble in water ; however, it is 

 but slightly soluble in hot water or brine. Owing to its great weight, 

 chlorine may be directly collected in a dry vessel by introducing the 

 gas-conducting tube into the bottom of the vessel. The chlorine will 

 lie in a heavy layer at the bottom of the vessel, displace the air, and 

 the extent to which it fills the vessel may be followed by its colour. 6 



4 The following proportions are then taken by weight : 8 parts of powdered manga- 

 nese peroxide, 4 parts of salt (best fused, to prevent its frothing), and 9 parts of sulphuric- 

 acid previously mixed with 5 parts of water. It is heated in a salt bath, so as to obtain a 

 temperature above 100. The corks in the apparatus must be soaked in paraffin (other- 

 wise they are corroded by the chlorine;, and black india-rubber tubing must be taken, and 

 not vulcanised (which contains sulphur, and becomes brittle under the act i< m of the chlorine). 

 The reaction which proceeds may be expressed thus: MiiO.^, ; '2XaCl + 2H.>SO4 = 

 MnSO 4 + NaoSO 4 -f 2H. 2 O + C1 2 . The preparation of C1. 2 from manganese peroxide and 

 hydrochloric acid was discovered by Scheele, and from sodium chloride by Berthollet. 



6 The reaction of hydrochloric acid on bleaching powder is very violent if all the 

 acid be added at once ; it should be poured in drop by drop (Merme, Kammerer). 

 C. Winkler proposed mixing bleaching powder with one quarter of burnt and powdered 

 gypsum, and having damped the mixture with water, to press aiad cut it up into cubes and 

 dry at the ordinary temperature. These cubes can be used for the preparation of chlorine 

 in the same apparatus as that used for the evolution of hydrogen and carbonic anhydride 

 the disengagement of the chlorine proceeds uniformly. 



A mixture of potassium dichromate and hydrochloric acid evolves chlorine perfectly 

 free from oxygen (V. Meyer and Langer). 



6 Chlorine is manufactured on a large scale from manganese peroxide and hydrochloric- 

 acid. It is most conveniently prepared in the arrangement shown in fig. (57, which con- 

 sists of a three-necked earthenware vessel whose central orifice 

 is the largest. A clay or lead funnel, furnished with a number 

 of orifices, is placed in the central wide neck of the vess.-l. 

 Roughly-ground lumps of natural manganese peroxide are placed 

 in the funnel, which is then closed by the cover N, and luted 

 with clay. One orifice is closed by a clay stopper, and is used for 

 the introduction of the hydrochloric acid and withdrawal of the 

 residues. The chlorine disengaged passes along a leaden gas- 

 conducting tube placed in the other orifice. A row of these 

 vessels is surrounded by a water-bath, to ensure their being 

 uniforml y heated. Manganese chloride is found in the residue. 

 rineon a large scale. In Weldon's process lime is added to the acid solution of man- 

 ganese chloride. A double decomposition takes place, resulting 



in the formation of manganous hydroxide and calcium chloride. When the insoluble 

 manganous hydroxide has settled, a further excess of milk of lime is added (to make 

 a mixture 2Mn(OH) 2 + CaO + xCaCl.>, which is found to be the best proportion, judging 

 from experiment), and then air is forced through the mixture. The hydroxide is then 

 converted from a colourless to a brown substance, containing peroxide, MnOo, and oxide 

 of manganese, Mn 2 Oj. This is due to the manganous oxide absorbing oxygen from the 



