459 



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 high temperature. Sainte-Claire Deville 

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

 tube (as with CO, Chapter IX.) covered with an amalgam of silver absorbs 

 chlorine from hydrochloric acid in a red-hot tube, and the escaping 

 gas contains hydrogen. V Meyer and Langer (1885) observed the de- 

 composition of hydrochloric acid at 1,690 in a platinum vessel , the 

 decomposition in this instance was proved not only from the fact 

 that hydrogen diffused through the platinum (p. 142), 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 agents. 2bi8 



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

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

 todium, A strong solution of hydrochloric acid is decomposed into equal volumes of 

 chlorine and hydrogen by the action of an electric current. If sodium chloride and lead 

 be melted in a crucible, the former being connected with the cathode and a. carbon anode 

 immersed in the lead, then the lead dissolves sodium and chlorine is disengaged as gas. 

 This electrolytic method has not yet been practised on a large scale, probably because 

 gaseous chlorine has not many applications, and because of the difficulty there is in 

 dealing with it. 



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

 and Meyev employed the dense g&iphitoidal carbon from gas retorts, and a powerful 

 blast. They determined the temperature by the alteration of the volume of nitrogen 

 In the platinum vessel, for this gas does not permeate through platinum, and is unaltered 

 by heat. 



bu 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. Hence 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 hy4ro- 

 Chloric acid with hydrogen, Lavoisier and Berthollet supposed it to be a compound with 

 oxygen of a anhydride contained in hydrochloric acid. They suppossd that hydro- 



