66 WRITINGS OF JAMES SMITHSON. 



nitrate of silver, a brick-red precipitate of arseniate of 

 silver. 



The white precipitate by muriate of barytes was only 

 partially soluble in nitric acid. The insoluble part of this 

 precipitate, of which the quantity was so minute that no 

 balance would have been sensible to it, was carefully col- 

 lected on to a very small bit of charcoal fixed to a pin. It 

 was then strongly heated at the blow-pipe. This bit of 

 charcoal now put into a drop of water, placed on a silver 

 coin, immediately made a black stain of sulphuret of silver 

 on the coin. This is the nicest test I am acquainted with 

 of the presence of sulphur, or sulphuric acid, in bodies. 



The quantity I possessed of this mineral for experiment 

 was very small. The above trials were made with particles 

 little more than visible ; however, the results, I think, suffi- 

 ciently establish the nature of the constituent parts. Their 

 respective proportions must remain for inquiries on another 

 scale. 



From Thomson's Annals of Philosophy, Vol. XVI, 1820, p. 100. 



Compound of Sulphuret of Lead and Arsenic. This is a new mineral 

 species discovered by Mr. Smithson, and described by him in the Annals of 

 Philosophy, xiv. 96. It was found in a magnesian lime rock in the Upper 

 Valais. It has a metallic aspect, a grey colour, and a fracture in some 

 directions vitreous, in others foliated. When triturated, yields a red pow- 

 der. Mr. Smithson, by a set of very minute but satisfactory experiments, 

 demonstrated that its constituents were sulphur, arsenic, and lead. 



