454 PRINCIPLES OF CHEMISTRY 



and that chlorine was a higher degree of oxidation of this radicle 

 murias (from the Latin name of hydrochloric acid, aciduni niuriati- 

 cum). It was only in 1811 that Gay-Lussac and Thenard in France 

 and Davy in England arrived at the conclusion that the substance 

 obtained by Scheele does not contain oxygen, nor under any conditions, 

 give water with hydrogen, and that there is no water in hydrochloric 

 acid gas, and therefore concluded that chlorine is an elementary 

 substance. They named it * chlorine ' from the Greek word ;(Aoy)09, 

 signifying a green colour, because of the peculiar colour by which this 

 gas is characterised. 



An aqueous solution of hydrochloric acid is generally employed for 

 the evolution of chlorine. The hydrogen has to be abstracted from the 

 hydrochloric acid. This is accomplished by nearly all oxidising 

 substances, and especially by those which are able to evolve oxygen at 

 a red heat (besides bases, such as mercury and silver oxides, which are 

 able to give salts with hydrogen chloride); for example, manganese 

 peroxide, potassium chlorate, chromic acid, itc. The decomposition 

 essentially consists in the oxygen of the oxidising substance displacing 

 the chlorine from 2HC1, forming water, H. 2 O, or taking up the hydrogen 

 and setting the chlorine free, as is sometimes said, 2HC1 + O (disengaged 

 by the oxidising substances) =H 2 O + C1 2 . Even nitric acid partially 

 produces a like reaction, but as we shall afterwards see its action is 

 more complicated, and it is therefore not suitable for the preparation of 

 pure chlorine. But other oxidising substances which do not give any 

 other volatile products with hydrochloric acid may be employed for 

 the preparation of chlorine. Among these may be mentioned : 

 potassium chlorate, acid potassium chromate, sodium manganate, 

 manganese peroxide, &c. Manganese peroxide is commonly employed 

 in the laboratory, and on a large scale, for the preparation of chlorine. 

 The chemical process which proceeds in this case may be represented 

 as follows : an exchange takes place between 4HC1 and MnO 2 , in 

 which the manganese takes the place of the four atoms of hydrogen, or 

 the chlorine and oxygen exchange places that is, MnCl, and 2H 2 O are 

 produced. The chlorine compound, MnCl 4 , obtained is very unstable ; 

 it splits up into chlorine, which as a gas passes from the sphere of 

 action, and a lower compound containing less chlorine than the 

 substance first formed, and which remains in the apparatus in which 

 the mixture is heated, MnCl t = MnCL 2 + Cl.,. 3 The action of hydro- 



3 This representation of the process of the reaction is the most natural. However,, 

 this decomposition is generally represented as if chlorine gave only one degree of comlii 

 nation with manganese, MnCLj, and therefore directly reacts in the following manner 

 .^ + 4HG1 = MnCL + 2ILO + Clo, in which case it is, as it wen-, supposed that 



