158 
MR. DAVY ON A SIMPLE ELECTRO-CHEMICAL METHOD 
or the finger. It is also presently dissolved by the aquafortis and muriatic acid 
of commerce, and by strong nitric acid. 
I coated one side of a platina spatula with gold, mixed a drop of acetate of 
lead with one of diluted aquafortis, and applied the zinc ; when the lead being 
reduced, the surface was washed, and by slight friction most of the lead was 
removed ; but there remained sufficient to show the colour and lustre of the 
metal. It dissolved by diluted aqua regia, leaving the surface of gold appa- 
rently unaltered. 
Compounds of copper. — The soluble salts of copper, as the perchloride, per- 
muriate, sulphate, nitrate, &c. when dissolved in water, require only to be 
placed on platina, and the zinc applied, when the copper will be presently re- 
duced and cover the platina. The compounds insoluble in water, as the proto- 
cldoride, proto-muriate, the carbonate, oxides, &c. when previously mixed with 
a little muriatic acid, or diluted aquafortis, are also readily reduced. The last 
acid seems best adapted in the case of different pigments of copper, as Olym- 
pian and Scheele’s greens, common verdigris, refiner’s verditer, &c. 
The copper reduced on platina, by zinc, usually exhibits the colour and 
lustre of the metal. Its surface is in some cases blackish, and requires gentle 
rubbing to show the metallic appearance. It has, in general, greater lustre, 
in cases when diluted aquafortis is used, and the acid only in slight excess 
when the zinc is applied. It is soluble in muriatic acid, but more readily in 
strong or diluted aquafortis. 
4. Experimental evidences that the accuracy of the Electro-chemical method of 
detecting metals is not impaired by the presence of mixed vegetable and 
animal substances, when applied to the detection of metallic poisons ; par- 
ticularly to the common compounds of Arsenic, Mercury, Lead, and Copper, 
in such mixtures. 
I have ascertained that the electro-chemical method is competent to the 
detection of very minute quantities of the different metals, when their com- 
pounds are mixed with vegetable and animal substances. But the object of 
this section is to notice briefly a number of miscellaneous experiments on the 
principal well known poisonous compounds of arsenic, mercury, lead, and 
copper, when mingled with organic fluids and solids, &c. 
