OF ASCERTAINING THE PRESENCE OF DIFFERENT METALS. 
155 
known oxide of arsenic, but of the metal. To which may be added the facts, 
that I have repeatedly coated the arsenic, deposited on platina, with copper, 
and also with mercury ; and removed both, without affecting the arsenic. All 
these circumstances seem to prove the accuracy of the statements made on the 
subject. 
Though zinc and platina were the metals used in the electro-chemical ex- 
periments on arsenic already noticed, I have also employed several other 
metals which generally afforded analogous results. Other metals might of 
course be substituted for zinc and platina, but as far as my experiments have 
extended I give these a decided preference. Iron and tin are much slower in 
their operation than zinc. Brass and copper are readily coated with metallic 
arsenic ; but the objections to the use of the common metals as substitutes for 
platina, arise from the facility with which they are acted on by heat, air, acids, 
&c. and the difficulty with which arsenic is separated from them without in- 
juring their surfaces. The advantage of colour which gold has over platina, 
when a grey metal, as arsenic, is to be reduced in contact with it, is cheaply 
purchased by simply gilding part of the inside of the platina crucible. 
Compounds of mercury . — The compounds of mercury in general are readily 
reduced by being placed on a slip of platina foil, mixed with a drop of diluted * 
aquafortis or muriatic acid, and the zinc applied, when the mercury is soon 
either partly attached loosely, or amalgamated with the platina, and partly 
combined with the zinc. This is the case with the black and red oxides, white 
precipitate, the acetate, subsulphate, cyanuret, fulminate, &c. The cyanuret, 
however, appears to be reduced more readily, and with greater lustre, when 
muriatic acid, undiluted, is employed, and the acetate and subsulphate best, 
when diluted aquafortis is used. The compounds of mercury soluble in water, 
in general require not the addition of acid. 
From the importance of the compounds of mercury with chlorine, our de- 
tails respecting them may be more minute. 
Corrosive sublimate . — If a drop of an aqueous solution of corrosive subli- 
mate be put on a bright surface of copper, it will soon, as is well known, ren- 
der the copper of a greyish black colour. If a similar experiment be made and 
* By <f diluted ” as applied to aquafortis and muriatic acid in this paper, is meant, the strong acids 
of commerce, to which an equal bulk of water has been added. 
X 2 
