472 PRINCIPLES OF CHEMISTRY 



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

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

 It is also remarkable that the presence of traces of certain substances, 23 

 especially of iodine, aluminium chloride, antimony chloride, &c., promotes 

 the action. 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. 25 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 + 01^ as CHgCl + HOI 



Marsh gas Chlorine Methyl chloride Hydrochloric acid 



The volume of the mixture remains unaltered. The methyl 

 chloride which is formed is a gas.- If it be separated from the hydro- 

 chloric acid (it is soluble in acetic acid, in which hydrochloric acid is but 

 sparingly soluble) and be again mixed with chlorine, then it may be 



85 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 

 ehowed 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, CgHg, 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 metalepsis. Gustavson showed that the modus operancli of this instructive reaction 

 is based on the property of aluminium bromide to enter into combination with hydro- 

 carbons 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. 



84 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 principle 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 an evolution of heat, but iij a 

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

 acids. Thus the reaction C 2 H 6 +Cl a =C 2 H 5 Cl+HCl, according to the data given by 

 Thomsen, evolves about 20,000 heat units, whilst the formation of hydrochloric acid 

 evolves 22,000 units 



