76 WRITINGS OF JAMES SMITHSON. 



doing this is important, are those of poisonings. In these 

 it is often of the first moment to be able to pronounce with 

 certainty, from portions of matter of extreme minuteness, 

 on the existence and the nature of the poison. 



Of Arsenic. 



I have already communicated the method here proposed 

 for the discovery of arsenic by employing it in the analysis 

 of the compound sulphuret of lead and arsenic from Upper 

 Yalais, printed in the Annals of Philosophy for August, 

 1819, but not having mentioned the" generality of its appli- 

 cation, or the great accuracy of it, it seems not superfluous, 

 from the importance of the subject, to resume it. 



If arsenic, or any of its compounds, is fused with nitrate 

 of potash, arseniate of potash is produced, of which the 

 solution affords a brick-red precipitate with nitrate of silver. 



In cases where any sensible portion of the potash of the 

 nitre has become set free, it must be saturated with acetous 

 acid, and the saline mixture dried and redissolved in water. 



So small is the quantity of arsenic required for this mode 

 of trial, that a drop of a solution of oxide of arsenic in 

 water, which, at a heat of 54.5 Fahr. contains not above 

 l-80th of oxide of arsenic,* put to nitrate of potash in the 

 platina spoon and fused, affords a considerable quantity of 

 arseniate of silver. Hence when no solid particle of oxide 

 of arsenic can be obtained, the presence of it may be estab- 

 lished by infusing in water the matters which contain it. 



The degree in which this test is sensible is readily deter- 

 mined. 



With 5.2 grains of silver, I obtained 6.4 grains of arse- 

 niate of silver ; but 0.65 grain of silver was recovered from 

 the liquors, so that the arseniate had been furnished by 4.55 

 grs. of silver. 



In a second trial 7.7 grains of silver, but of which only 

 6.8 grains precipitated, yielded 9.5 grs. of arseniate. 



*Chimie de Thenard, ii, p. 167. 



