150 
MR. DAVY ON A SIMPLE ELECTRO-CHEMICAL METHOD 
respective metal deposited on the platina. These are a few instances of the 
apparent reduction of metallic salts on platina by the agency of Voltaic elec- 
tricity. It would be extremely easy to extend the list to an indefinite number 
of other cases, did I not fear to anticipate details which more properly belong 
to the subsequent part of the subject. 
The compounds of arsenic, mercury, lead and copper afford the principal 
metallic poisons ; and a knowledge of their properties, and of the best means 
of detecting them, derives interest from many considerations, but is particu- 
larly valuable from the paramount importance justly attached to chemical 
evidence in cases of accidental or intentional poisoning. I shall proceed to 
offer proofs of the efficacy of the electro-chemical method to detect the com- 
pounds of those metals in the order in which they have been enumerated. 
Greyish black arsenic, protoxide of Berzelius, or 
fly-powder. 
.White oxide of arsenic, or arsenious acid. 
When half a grain, or less, of either of those solid compounds is placed 
on a slip of platina foil, mixed with a drop or two of muriatic acid *, and 
the zinc applied^, the arsenic will presently be reduced to the metallic 
state ; one part will be deposited on the platina, and the other part mixed in 
thin filaments with the fluid. The surface of the platina will become iridescent 
or exhibit variegated colours (resembling heated steel), as blue with tints of 
red, yellow &c. Much of the arsenic is thus strongly attached to the platina, 
and cannot be removed by wiping or rubbing it with the finger or a cloth, nor 
by cold strong muriatic or sulphuric acid, nor by hot solutions of caustic 
alkalies ; but it instantly disappears on being touched with the smallest drop of 
Compounds of Arsenic . — < 
» By “ muriatic acid ” in this paper, the common strong acid of commerce is meant. I have used it 
in preference to the pure acid, from the facility with which it can be procured. 
f By the words ” zinc applied,” used here and in other places, is meant bringing the zinc in contact 
both with the platina and the substances to be acted on ; or moving the zinc about on the platina, which 
seems the readiest way of effecting the reduction of many metallic compounds. 
Diluted muriatic acid occasions analogous effects, but they are more slowly produced, probably be- 
cause the undiluted acid is a better solvent of the arsenical compounds, and a better conductor of elec- 
tricity. Like effects occur with strong sulphuric acid, if the zinc be moved about on the platina, but 
they are not produced with strong nitric acid or aquafortis, and only very gradually by these acids when 
diluted. 
