•122 On Nitrat of Silver f 



All the particulars respecting this mode of detection 

 having b^'cn fully stated b) Dr. Kogei, with such references 

 to fanner writers on the subject as tlie ci'.se required, it 

 would be quite siipeifuious to enter into anv further detail 

 on this head. My object in resuming the subject, the 

 practical importance of which need not be pointed out, is 

 to coninumicate to the Society the result of an inquiry 

 •which 1 have made on the nature of the yellow precipitate, 

 the appearance of which is assumed as denoting the pre- 

 sence of arsenic, and to answer some objections which have 

 been made against this test by Mr. Sylvester, of Derby, in 

 a paper on metallic poisons, recently published in Nichol- 

 son's Journal* . 



The yellow compound in question has the following 

 properties : 



If, after bein<; well washed with distilled water, it he 

 suffered to stand for some time in an open vessel, it gra- 

 <lually passes to a brown colour ; but it does not, like ni- 

 trat 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 ; but after it has been separated and dried, it is 

 no longer sensibly 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, in the form of a glo- 

 bule, IS still more distinct and striking, if a little carbona- 

 ceous matter be mixed with the precipitate, and the blow- 

 pipe applied. 



When the yellow precipitate, inclosed in a tube, is ex- 

 posed to the heat of a lamp, the while smoke condenses 

 on the cold part of the tube, in minute octohedral crystals 

 of arscnlous acid. 



It appears, therefore, that the precipitate in question is a 

 combination of white arsenic (arsenious acid) and silver, or 

 an arsenite of silver; and it is inferred that its formation, 

 when ammonia and nitrat of silver are added to a mixture 

 containing arsenious acid, is owing to a double elective de- 

 composition of the arsenite of ammonia by the nitrat of 

 silver, in consequence of which arsenite of silver is formed, 

 and separates as an insoluble precipitate from the nitrat of 

 ammonia which remains in the solution. The addition of 

 annnonia is necessary, because arsenic acid alone cannot 



* KJcIidhon s Juurnal for December 18)2, vol. xixiii. p. 306. 



decompose 



