OF ASCERTAINING THE PRESENCE OF DIFFERENT METALS. 163 
reduce the lead, but as a slight excess of acid will redissolve the lead (partially 
or wholly), a longer contact of the zinc is necessary to neutralize the acid, and 
render the lead permanent. 
Sulphate of copper, blue vitriol . — A small quantity of sulphate of copper in 
solution was added to the following substances, viz. flour forming a soft paste, 
milk, tea and coffee sweetened with sugar and cream, gelatine (isinglass) in 
solution, and with nutgalls, albumen (white of egg) and yolk of egg, saliva, 
sheep’s blood, and ox-bile. On a little of each mixture being placed on the 
platina spatula, and the zinc applied, the copper was presently reduced on the 
platina. In some cases, the surface was of a blackish colour, but its proper 
colour and lustre became apparent on rubbing it. The addition of a little 
diluted aquafortis or muriatic acid to the mixtures, usually facilitated the re- 
duction of the copper. 
A little sulphate of copper in powder was mixed with butter and lard, placed 
on the spatula, and a drop of diluted aquafortis incorporated with them. On 
the zinc being applied, the copper was readily reduced. 
5. Some general Remarks, and Conclusion. 
The experiments detailed in this paper seem to prove, that the common com- 
pounds of arsenic, mercury, lead, and copper, may be readily reduced to the 
metallic state, on platina, by the electro-chemical method described ; and that 
this method is also competent to the detection of those metals, in cases when 
their compounds are mixed in very small quantity with vegetable and animal 
substances. It may, I think, be further deduced, that the method is applicable 
in cases when those compounds exist in the most complicated mixtures of 
organic substances. For, though I have had no opportunity of applying it to 
the contents and tissues of the stomach in instances of poisoning, yet its 
efficacy in such cases can scarcely be doubted, as no animal or vegetable sub- 
stances can, I apprehend, resist the action of the mineral acids, which are 
almost indispensably necessary to the success of the method. In some instances, 
where the common tests will not act at all, and in others, where they only act 
fallaciously, the electro-chemical method will, in general, be found to act with 
certainty. 
The general results appear to be strictly electro-chemical, or to arise from 
y 2 
