152 
MR. DAVY ON A SIMPLE ELECTRO-CHEMICAL METHOD 
pelled, as a minute quantity of this acid retards the reduction of the arsenic, 
or re-dissolves it. 
The metallic arsenites and arseniates, where two metals are present, appear 
also in general to require a modified treatment, according to the object in 
view, and seem to offer proofs of the elegance of this mode of detecting 
metals. To give an instance or two, in the case of the arsenite or arseniate of 
copper. If a little of either of these compounds be dissolved in a few drops of 
muriatic acid, by heat, in the platina spoon, and a few tolerably quick contacts 
be made with zinc foil, the arsenic only will be reduced ; part of it will be 
deposited on the spoon, and part will remain in the fluid as a dark grey or 
blackish substance. 
If a little of either of the above compounds be dissolved in a few drops of 
diluted nitric acid, (consisting of one volume strong acid to three of water,) 
in the platina spoon ; boiled nearly to dryness, and water added just enough 
to obtain a solution ; if the point of a slip of zinc be now applied to the centre 
of the bulb of the spoon, the copper alone will presently be reduced on the 
platina, exhibiting an unusual metallic lustre, and forming a circle round the 
zinc. If the zinc be now removed, the copper will be re-dissolved by the slight 
excess of acid present. If the contact of the zinc be continued, in a short time 
the arsenic will form a circle on the platina, round the zinc, whilst beyond it 
the copper will make its appearance. 
I found by repeated experiments, that the arsenic precipitated by zinc on 
platina foil from solid arsenious acid, or from arsenical solutions, and muriatic 
acid, could be readily obtained in the form of arsenious acid, by coiling the 
dried foil, and exposing it to the heat of a spirit lamp, in a tube closed at one 
end, or open at both, and stopped with a cork or corks, so as to allow the ex- 
panded air to escape. 
With a view to gain some approximation as to the actual quantity of arsenic 
that could be detected by the electro-chemical method, and to ascertain if it 
could be procured from the platina in the metallic state, I placed on a new 
slip of platina weighing 22.14 grains, five drops of an aqueous solution 
of arsenious acid, and three drops of muriatic acid ; a slip of zinc being 
applied, the arsenic was soon reduced, and much of it adhered to the pla- 
tina, which, after being washed in pure water and dried, acquired an in- 
