of the Cacodyl Series. 391 



half of the pure gaseous mixture, which contains one volume 

 of olefiant gas, and two volumes (C H2) of marsh gas, must, 

 in fact, upon burning with three and a half volumes of oxygen, 

 produce two volumes of carbonic acid. 



Whilst the absence of arsenietted hydrogen and free hy- 

 drogen decidedly proves that the first is not to be considered 

 as a constituent element of cacodyl; the conclusion may be 

 drawn, at the same time, from these appearances of decom- 

 position, that if the radical C4 H^j can exist independently, it 

 is most unstable and is decomposed much below a red heat. 



Among the products of the decomposition of cacodyl, there 

 is one substance which I have mentioned several times, and 

 to which I have given the arbitrary name of eri/trarsin. I shall 

 now consider this substance, as it is in close connexion with 

 the foregoing substances. I have not hitherto succeeded in 

 obtaining any quantity of this remarkable substance. It is 

 formed as a secondary product in the formation of chloride of 

 cacodyl, sometimes in a great and sometimes in a small quan- 

 tity. It is also deposited upon the distillation of oxide of ca- 

 codyl with water. Upon conducting the vapour of cacodyl, 

 or oxide of cacodyl, through tubes slightly heated, this sub- 

 stance is produced in large quantities by an imperfect com- 

 bustion ; but, obtained in this manner, it is always contami- 

 nated with arsenic, from which it is impossible to separate it. 

 The substance next made use of in preparing it was obtained 

 in the following manner. 



About 100 grammes of oxide of cacodyl was added to con- 

 centrated hydrochloric acid ; chloride of cacodyl was formed, 

 and a red flocculent precipitate fell, which, after distillation of 

 the chloride, remains behind in the retort. The precipitate 

 became during the distillation of a thick consistence, which 

 increased, and became of a darker colour, with the appearance 

 of finely divided red oxide of iron. After six or eight boilings 

 with absolute alcohol, the substance was obtained quite pure 

 and free from chlorine. It is necessary during this boiling to 

 protect it from the air and to dry the substance in a vacuum 

 with sulplnnic acid, as otherwise it is liable to absorb oxygen 

 slowly. Prepared in this manner, erytrarsin is of a steel blue 

 shading into dark red, free from smell, and without the least 

 appearance of crystallization. It is easily rubbed down into 

 a red powder, which absorbs oxygen slowly from the air, with 

 the appearance of the formation of arsenious acid, as it be- 

 comes covered with a white powder. This decomposition 

 does not take place until after exposure for several weeks. It 

 is not soluble in alcohol, jcther or water — even caustic potash 

 does not act upon it. In concentrated and not fuming nitric 



