AND KESEARCHES OF JAMES SMITHSON. 133 



"If arsenic, or any of its compounds, is fused with the 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 be- 

 come 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 

 deg., Fahr., contains not above -fa of oxide of arsenic, put to nitrate of 

 potash, in the platina spoon, and fused, affords a considerable quantity of 

 arseniate of silver. Hence, whence no solid particles of .oxide of arsenic 

 can be obtained, the presence of it may be established by infusing in water 

 the matter which contains it. 



" The degree in which this test is sensible is readily determined. 



" With 5 2 grains of silver he obtained 6.4 grains of arseniate of silver j 

 but 0.65 grains of silver was recovered from the liquors, so that the arsen- 

 iate had been furnished by 4.55 grains of silver. In a second trial, 7.7 

 grains, of which only 6.8 grains precipitated, yielded 9.5 grains of arseniate. 

 The mean is 140.17 from 100 silver." 



Before the invention of the method of subliming a ring 

 of arsenic in a glass tube, and that more recently employed 

 by Marsh, of converting it, by means of hydrogen, into ar- 

 seniuretted hydrogen, the method of Smithson was among 

 the most delicate in use, and, as a means of obtaining col- 

 lateral evidence of the presence of arsenic, it still continues 

 to be employed. 



With respect to mercury, he remarks : 



" All the oxides and saline compounds of mercury laid in a drop of ma- 

 rine acid, on gold, with a bit of tin, quickly amalgamate the gold. 



u A particle of the corrosive sublimate, or a drop of a solution of it may 

 be thus tried. The addition of marine acid is not required in this case. 

 Quantities of mercury may be rendered evident in this way which could 

 not be so by any other means." 



This test for mercury, it may be remarked, still keeps its 

 place among the best evidences of the presence of that 

 metal. 



" This method will exhibit the mercury in cinnabar. It must be pre- 

 viously boiled with sulphuric acid, in the platina spoon, to convert it into 

 sulphate." 



" Cinnabar heated in a solution of potash, on gold, amalgamates it." 

 " A most minute quantity of metallic mercury may be discovered, in a 

 powder, by placing it in nitric acid, on gold, drying, and adding muriatic 

 acid and tin." 



14. In the same volume (xx.) is, at page 363, a letter to 

 the editor of the Annals, On some improvements on lamps, 

 particularly referring to the form of the wicks, the employment 

 of wax as their fuel, and the mode of extinguishing them, 

 by putting sound wax to the wicks, and then blowing out 

 the flame. 



" It is to be regretted," remarks the author, " that those who cultivate 

 science, frequently withhold improvements in their apparatus and processes, 

 from which they themselves derive advantage, owing to their not deem- 



