SEPARATION OF ARSENIC. 
63 
poured into an apparatus where there should be already a 
strong disengagement of hydrogen. The result will be im- 
mediately apparent. 
OBSERVATIONS OF M. LIEBIG. 
The sensibility of the method pointed out, for the discovery 
of arsenic in a liquid which contains it in the state of arsenious 
acid, almost surpasses imagination. The flame of hydrogen 
gas disengaged from zinc and hydrochloric acid, to which 
there has been added 0077 of a grain of arsenious acid, will 
cover, upon a fair and bright plate of porcelain, a surface of 
half an inch square with a black glistening metallic film of 
arsenic. 
We may, however, be exposed to extremely fatal mistakes, 
when the liquid upon which we make our researches contains 
foreign metals. If, for example, we dissolve pure iron in 
hydrochloric acid, and direct the flame of the disengaged 
hydrogen gas upon a porcelain surface, this will always be 
covered with a strong black film, which we are liable to take 
for arsenic, but which is nothing else but metallic iron. In 
fact, the gas, while passing off, carries with it some extremely 
fine drops of the solution, and the chloride contained in it is 
reduced by the flame; the metallic iron deposited on the 
porcelain, burns at the edges of the flame, and is changed into 
oxide; the film of arsenic, moreover, is distinguished easily; 
it disappears immediately when moistened with a drop of 
nitric acid, or hydrosulphate of ammonia, while that of iron 
is not attacked by the acid, and is colored greenish black by 
the hydrosulphate. This phenomenon is manifest, likewise, 
when we pass the gas through a large glass tube, a foot in 
length, and filled with hydrate of potassa in large fragments. 
The proof is rather more certain when we pass the gas 
through a long tube filled with cotton slightly pressed. All the 
heavy metals, and among the rest antimony especially, act 
like iron when they are mixed with the solutions. 
The method of Mr. Marsh presents complete certainty 
