453 



CHAPTER XI 



THE HALOGENS : CHLORINE, BROMINE, IODINE, AND FLUORINE 



ALTHOUGH hydrochloric acid, like water, is one of the most stable 

 substances, it is nevertheless decomposed not only by the action of a 

 .galvanic current l but also by a rise of temperature. Sainte-Claire Deville 

 showed that decomposition already occurs at 1 300, because a cold tube 

 (p. 388) covered with an amalgam of silver absorbs chlorine in a red- 

 hot tube, and the escaping gas contains hydrogen. Meyer and Langer 

 (1885) observed the decomposition of hydrochloric acid at 1690 in a 

 platinum vessel ; the decomposition in this instance was proved not 

 only from the fact that hydrogen permeated through the platinum 

 (p. 141), owing to which the volume was diminished, but also from 

 chlorine being obtained in the residue (the. hydrogen chloride was 

 mixed with nitrogen), which liberated iodine from potassium iodide. 2 

 The usual method for the preparation of chlorine consists in the 

 abstraction of the hydrogen by oxidising agencies. 



The acid properties of hydrochloric acid were known when 

 Lavoisier pointed out the formation of acids by the combination of 

 water with the oxides of the non-metals, and therefore there was reason 

 for thinking that hydrochloric acid was formed by the combination of 

 water with the oxide of some element. Therefore when Scheele 

 obtained chlorine by the action of hydrochloric acid on manganese 

 peroxide he considered it as the acid contained in common salt. When 

 it became known that chlorine gives hydrochloric acid with hydrogen, 

 Lavoisier and Berthollet supposed it to be a compound with oxygen of 

 <in anhydride contained in hydrochloric acid. They supposed that 

 hydrochloric acid contained water and the oxide of a particular radicle 



1 The decomposition of fused sodium chloride by an electric current has been proposed 

 in America and Russia (N. X. Beketoff) as a means for the preparation of chlorine and 

 sodium. A strong solution of hydrochloric acid is decomposed into equal volumes of 

 chlorine and hydrogen by the action of an electric current. 



2 To obtain so high a temperature (at which the best kinds of porcelain soften) Langer 

 .Did Meyer employed the dense graphitoidal carbon from gas retorts, and a strong draught. 

 They determined the temperature by, the alteration of the volume of nitrogen in the 

 platinum vessel, for it does not permeate through platinum, and is not altered by heat. 



