466 PRINCIPLES OF CHEMISTRY 



Or, in general terms 



RH + Cl, = RC1 + HC1. 



The conditions under which metalepsis takes place are also very 

 constant. In the dark chlorine does not usually act on hydrogen com- 

 pounds, but the action commences under the influence of light. The 

 direct action of the sun's rays is particularly propitious to metalepsis. 

 It is also remarkable that the presence of traces of certain substances 2 ^ 

 promotes the action (especially of iodine, aluminium chloride, antimony 

 chloride, &c.). A trace of iodine added to the substance subjected to- 

 metalepsis often produces the same effect as sunlight. 24 



If marsh gas be mixed with chlorine and the mixture ignited, then 

 the hydrogen is entirely taken up from the marsh gas and hydrochloric- 

 acid and carbon formed, but there is no metalepsis. 2fi But if a 

 mixture of equal volumes of chlorine and marsh gas be exposed to the 

 action of diffused light, then the greenish yellow mixture gradually 

 becomes colourless, and hydrochloric acid and the first product of 

 metalepsis, namely, methyl chloride, are formed : 



CH 4 + C1 2 = CH 3 C1 + HC1 



Marsh gas. Chlorine. Methyl chlorii 1 >. Hydrochloric acid. 



25 Such carriers or media for the transference of chlorine and the halogens in general' 

 were long known to exist in iodine and antimonious chloride, and have been most fully 

 studied by Gustavson and Friedel, of the Petroffsky Academy the former with respect 

 to aluminium bromide, and the latter with respect to aluminium chloride. Gustavson 

 showed that if a trace of metallic aluminium be dissolved in bromine (it floats on bromine,, 

 and when combination takes place much heat and light are evolved), the latter becomes 

 endowed with the property of entering into metalepsis, which it is not able to do of its 

 own accord. When pure, for instance, it acts very slowly on benzene, C 6 H 6 , but in the 

 presence of a trace of aluminium bromide the reaction proceeds violently and easily, so that 

 each drop of the hydrocarbon gives a mass of hydrobromic acid, and of the product of meta- 

 lepsis. Gustavson showed that the modus operandi of this instructive reaction is based 

 on the property of aluminium bromide to enter into combination with hydrocarbons and 

 their derivatives. The details of this and all researches concerning the metalepsis of 

 the hydrocarbons must be looked for in works on organic chemistry. 



24 As small admixtures of iodine, aluminium bromide, &c., aid the metalepsis of large 

 quantities of a substance, just as nitric oxide aids the reaction of sulphurous anhydride 

 on oxygen and water, so the matter is essentially the same in both cases. Effects of this- 

 kind (which should also be explained by a chemical reaction proceeding at the surfaces) 

 only differ from true contact phenomena in that the latter are produced by solid bodies 

 and are accomplished at their surfaces, whilst in the former all is in solution. Probably 

 the action of iodine is founded on the formation of iodine chloride, which reacts more 

 easily than chlorine. 



25 Metalepsis belongs to the number of delicate reactions if it may be so expressed 

 as compared with the energetic reaction of combustion. Many cases of substitution are 

 of this kind. Reactions of metalepsis are accompanied by the evolution of heat, but in a 

 less quantity than that evolved in the formation of the resulting quantity of the halogen 

 acids. Thus the reaction C 2 H 6 + C1 2 = C 2 H 3 C1 + HC1, judging from the data given by 



