154 
MR. DAVY ON A SIMPLE ELECTRO-CHEMICAL METHOD 
a drop of muriatic acid. A small slip of zinc being applied, the spot became 
of a dark steel-grey colour with a tint of green at its edge. Some pure water 
being put into the crucible, some extremely minute films of metallic arsenic 
appeared on the surface of it. The crucible, after being washed and dried, was 
heated, when the arsenic rose, leaving the gold surface unaltered. 
Into the crucible used in the preceding experiment, I put -foth of a grain of 
solid arsenious acid, and dissolved it in about five drops of muriatic acid. I 
then applied a slip of zinc for about half a minute ; as soon as the contact of 
the zinc was made, the variegated colours from the arsenic were beautifully 
produced at the bottom of the crucible, an effect which was succeeded by a 
violent action, and all the arsenic was reduced. The crucible was now filled 
with pure water, and numerous steel-grey coloured filaments of metallic arsenic 
floated on the surface ; some being collected on a slip of platina and heated, the 
alliaceous odour was strongly produced, and they were dissipated in a white 
vapour. The crucible being washed, and a few drops of diluted muriatic 
acid put into it, it was rinsed in pure water and dried, when its gold surface 
was so completely coated with arsenic of a dark steel-grey colour, that no 
vestige of the gold at the bottom of the crucible could be seen, even with the 
aid of a magnifying glass, though the surface was full of little inequalities. The 
arsenic on the crucible did not sensibly tarnish by exposure to the air for some 
days ; a portion of it was then expelled by a heat of nearly 400° Fahr., when 
the gold became partially visible ; the remainder of the arsenic continued of a 
steel-grey colour. 
Though it seems unnecessary to bring forward more experiments in proof of 
the efficacy of the electro-chemical method to detect arsenic, it may be proper 
to recur to the evidence that arsenical compounds are by this method reduced 
to the metallic state. The experiments with the platina gilt crucible afford the 
most unequivocal proofs of metallic arsenic ; whilst those with the platina foil 
and spoon, though seemingly more ambiguous, will, on examination, I presume, 
be found scarcely less satisfactory. Thus the partial oxidation of the surface 
exhibited in variegated colours ; the strong cohesion of the arsenic to the pla- 
tina, are characteristics of the metal. The alliaceous odour (in cases where 
no deoxygenating substance can be presumed to exist) is admitted to belong 
only to the metal ; insolubility in strong muriatic acid, is a property of no 
