THE HALOGENS 468 



Chlorine Is a gas of a yellowish green colour, and has a very 

 suffocating and characteristic odour. On lowering the temperature to 

 50 or increasing the pressure to six atmospheres (at 0) chlorine 

 condenses 7 into a liquid which has a yellowish-green colour, a 

 density of 1*3, and boils at 34 The density and atomic weight of 

 chlorine is 35-5 times greater than that of hydrogen, hence the molecule 

 contains C1 2 8 At one volume of water dissolves about 1^ volume 

 of chlorine, at 10 about 3 volumes, at 50 again \\ volume. 9 Such 



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 

 uniformly heated. Manganese chloride is found in the residue. 

 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+<rCaCl 2 , which is found to be the best propor- 

 tion, judging from experiment), and then air is forced through 

 the mixture. The hydroxide is thus converted from a colourless S 3 



to a brown substance, containing peroxide, MnO 2 , and oxide of chlorine on a large 

 manganese, Mn 2 O 5 . This is due to the manganous oxide absorb- scale - 

 ing oxygen from the air. Under the action of hydrochloric acid this mixture evolves 

 chlorine, because of all the compounds of chlorine and manganese the chloride MnCLj 

 is the only one which is stable (see Note 8). Thus one and the same mass of manganese 

 may be repeatedly used for the preparation of chlorine. The same result is attained in 

 other ways If manganous oxide be subjected to the action of oxides of nitrogen and air 

 (Coleman's process), then manganese nitrate is formed, which at a red heat gives oxides 

 of nitrogen (which are again used in the process) and manganese peroxide, which is thus 

 renewed for the fresh evolution of chlorine. 



7 Davy and Faraday liquefied chlorine in 1823 by heating the crystallo-hydrate 

 Cla8H 2 O in a bent tube (as with NH 3 ), surrounded by warm water, while the other end of 

 the tube was immersed in a freezing mixture. Meselan condensed chlorine in freshly-burnt 

 charcoal (placed in a glass tube), which when cold absorbs an equal weight of chlorine. 

 The tube was then fused up, the bent end cooled, and the charcoal heated, by which 

 means the chlorine was expelled from the charcoal, and the pressure increased. 



8 Judging from Ludwig's observations (1868), and from the fact that the coefficient of 

 expansion of gases increases with their molecular weight (Chapter II., Note 26, for hydrogen 

 -0-867, carbonic anhydride = 0-878, hydrogen bromide =0'886),it might be expected that 

 the expansion of chlorine would be greater than that of air or of the gases composing it. 

 V. Meyer and Langer (1885) having remarked that at 1,400 the density of chlorine (taking 

 its expansion as equal to that of nitrogen) =29, consider that the molecules of chlorine 

 split up and partially give molecules Cl, but it might be maintained that the decrease in 

 density observed only depends on the increase of the coefficient of expansion. 



9 Investigations on the solubility of chlorine in water (the solutions evolve all their 

 chlorine on boiling and passing air through them) show many different peculiarities. First 

 Gay-Lussac, and subsequently Pelouze, determined that the solubility increases between 

 and 8-10 (from 1 J to 2 vols. of chlorine per 100 vols. of water at up to 8 to 2| at 10). 

 In the following note we shall 'see that this is not due to the breaking- up of the hydrate at 

 about 8 to 10, but to its formation below 9. Roscoe observed an increase in the solu- 

 bility of chlorine in the presence of hydrogen even in the dark. Berthelot determined 



*8 



