OF ASCERTAINING THE PRESENCE OF DIFFERENT METALS. 
151 
strong nitric acid, or of the aquafortis of commerce. The arsenic is also rea- 
dily removed by exposing the foil to the moderate heat of a spirit lamp, when 
it rises in the form of arsenious acid ; but previous to this effect, the alliaceous, 
or garlick-like odour, so characteristic of this metal, is strongly produced *, 
and the surface of the platina remains unaltered. 
The following arsenical compounds, when exposed to similar treatment, 
afford analogous results, viz. 
Arsenious acid in solution. 
Arsenites of potash, lime, &c. 
Arsenic acid. 
Arseniates of potash, lime, &c. 
Chloride of arsenic. 
Sulphurets of arsenic, obtained by passing sulphuretted hydrogen gas through 
solutions of arsenious and arsenic acids. 
In cases where the quantity of any arsenical compound in solution is very 
minute, the fluid should be concentrated, or boiled nearly to dryness, previous 
to the addition of the muriatic acid, and subsequent trial on the platina foil by 
zinc. 
Some arsenical compounds require a treatment somewhat different from those 
already enumerated, in order to exhibit the arsenic in a satisfactory manner. 
This is the case with the native sulphurets of arsenic, which being scarcely 
acted on by muriatic acid, require previous treatment with nitric acid. Thus, 
a little realgar or orpiment, in powder, was mixed with a drop of strong nitric 
acid in the platina spoon, heated, and the excess of acid expelled ; a drop or 
two of muriatic acid being incorporated with the residual substance, the zinc 
was applied, and the arsenic readily reduced on the spoon, exhibiting its cha- 
racteristic appearance. Arsenical pyrites, too, requires the previous addition 
of nitric acid. In operating on such arsenical compounds as require the use 
of nitric acid, as little as possible should be employed, and any excess ex- 
* Dr. Christison, in his valuable work “ On Poisons,” proposes to discard this test altogether, chiefly, 
it would seem, from its being obscured entirely by the presence of a very small portion of vegetable or 
animal matter ; but as this objection does not apply to the electro-chemical mode of detecting arsenic, 
the alliaceous odour is regarded, especially in these experiments, as a striking character, exhibited by 
no other metal, as far as my experience extends. 
