THE HALOGENS 457 



Chlorine is a //</* of a yellowish green colour, and has a very 

 suffocating and characteristic odour. On lowering the temperature to 

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

 condenses 7 into a liquid which has a yellowish green colour and 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 H volumes 

 of chlorine, at 10 about three volumes, at 50 again 1^ volumes. 9 

 Such a solution of chlorine is termed ' chlorine water ' ; and is employed 

 in a diluted form in medicine and laboratory practice. It is prepared 



air. Under the action of hydrochloric acid this mixture evolves chlorine, because of all 

 the compounds of chlorine and manganese the chloride MnClo is the only one which is 

 staMe (see Note 3). 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 manga- 

 nous 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 

 CLMtLO in a bent tube (like that shown in fig. 45, p. 248), in warm water, while the other 

 nd 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 

 = 0'367, carbonic anhydride =0*373, hydrogen bromide 0'386),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(lSKf>) having remarked that at 1400 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 thought 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 (ray-Lussac, and then Pelouze, determined that the solubility increases between 

 and 8 to 10 3 (from l to 2 vols. of chlorine per 100 vols. of water atO 3 to 8 to 2J at 10). 

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

 about 8 J 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 

 an increase of solubility with the progress of time. Schb'nebein and others suppose that 

 chlorine arts <>n water, forming hypochlorous and hydrochloric acids fHClO + HCl). 



The equilibrium between chlorine, steam as a gas, between water, liquid chlorine, ice, 

 and the solid crystallo-hydrate of chlorine is evidently very complex. Guldberg (1870) 

 gave a theory for similar states of equilibrium, which was afterwards developed by Rooze- 

 boom (1887), but it would be inopportune here to enter into its details. It will be enough 

 in the first place to mention, that there is now no doubt (according to the theory of heat, 

 and the direct observations of Ramsay and Young) that the vapour tension at one and the 

 same temperature are different for the liquid and solid states of substances; secondly, to 

 call attention to the following note ; and, thirdly, to state that, in the presence of the 

 crystallo-hydrate, water between 0'24 and + 28'7 (when the hydrate and a solution may 

 occur simultaneously) dissolves a different amount of chlorine than it does in the absence 

 of the crystallo-hydrate. 



