460 PRINCIPLES OF CHEMISTRY 



A.n 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, &c The decomposition 

 essentially consists in the oxygen of the oxidising substance displacing 

 the chlorine from 2HC1. forming water, H 2 O, and setting the 

 chlorine free. 2HC1 -i- O (disengaged by the oxidising substances) 

 es H 2 O -f 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. 3 

 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, &o 

 Manganese peroxide is commonly employed in the laboratory, and on 

 a large scale, for the preparation of chlorine. The chemical process 

 in this case may be represented as follows an exchange takes place 

 between 4HC1 and Mn0 2 , in which the manganese takes the place 

 of the four atoms of hydrogen, or the chlorine/ and oxygen exchange 

 places that is, MnCl 4 and 2H 2 are produced. The chlorine com- 

 pound, 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, which remains 

 in the apparatus in which the mixture is heated, MnCl 4 = MnClg 

 f Cl 3 3bis The action of hydrochloric acid requires a temperature of 



chloric acid contained water and' the oxide of & particular radicle, and that chlorine was 

 a higher degree of oxidation of this radicle murias (from the Latin name of hydrochloric 

 acid, acidum muriaticum). 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 toy 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 xAwpfo, 

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

 terised. 



3 However, nitric acid has been proposed as a means for obtaining chlorine, but by 

 methods which have the drawback of being very complicated. 



5bil 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 combi- 

 nation with manganese, MnCl 2 , and therefore directly reacts in the following manner 

 MnO 2 + 4HC1 = ,MnCl 2 + 2H 2 + C1 2 , in which case it is supposed that manganese 

 peroxide, Mn6 2 , breaks up, as it were, into raanganous oxide, MnO, and oxygen, both of 

 which react with hydrochloric acid, the manganous oxide acting upon HC1 as a base, 

 giving MnCl 2 and at the same time 2HC1 + O = H 2 O + C1 8 . In reality, a mixture of oxygen 



