\ 



TEST FOR ARSENIC. 175 



its comparative meiits, we were induced to consider as the 



most effectual of all the te>ts hitherto used for that purpose. 



The method consists simply in adding, in succession, to the 



fluid suspected to contain arsenic, minute quantities of solutions 



of ammonia and of nitrate of silver ; by which means, if the 



smallest quantity of arsenic be present, a dense yellow pre- X e11 ? w P re *. 



. . n J l i • ■•■ cjp. it arsenic 



cipitate will be produced. be present* 



All the particular! respecting this mode of detection having 

 been fully stated by Dr. Roget, with such references to former 

 writers on the subject as the case required, it would be quite 

 supeifluous to enter into any further detail on this head. My 

 object in resuming the subject, the practical, importance of 

 which need not be pointed out, is to communicate to the So- 

 ciety the result of an inquiry which I have made on the nature 

 of the yellow precipitate, the appearance of which is assumed 

 as denoting the presence of arsenic, and to answer some objec- 

 tions which have been made against this test by Mr. Sylvester, Objections by 

 of Derby, in a paper on metallic poisons, recently published in l * y vestei - 

 Nicholson's Journal*. 



The yellow compound in question has the following pro- t he OP yeiiow° 

 perties : precipitate. 



If, after being well washed with distilled water, it be suf- 

 fered to stand for some time in an open vessel, it gradually 

 passes to a brown colour j but it does not, like nitrate of silver, 

 become black on continuing this exposure. 



It is readily soluble in dilute nitric acid./ It also dissolves on 

 adding an excess of ammonia at the moment of its formation j 

 but after it has been separated and dried, it is no longer sen- 

 sibly soluble in ammonia. 



If a small quantity of this precipitate be exposed to the heat 

 of a lamp on a slip of laminated platina, a white smoke arises 

 from it, and metallic silver remains attached to the platina.' 

 The reduction of the silver, id the form of a globule, is still 

 more distinct and striking, if a littl- carbonaceous matter be 

 mixed with the precipitate, and the blowpipe applied. 



When the yellow precipitate, inclosed in a tube, is exposed 

 to the heat of a lamp, the white smoke condenses on the cold ; 



* Nicholson's Journal for December, 1812. Vol. xxxiii. p. 1Q6. 



part 



