DETECTION OF ARSENIC IN ORGANIC MIXTURES. 223 
is, that a salt of mercury gives a mercurial coating to the 
zinc, and thus prevents the deposition of the arsenic, and 
the subsequent action of sulphuric acid upon it. It must 
however exist in very considerable quantity to offer any 
serious impediment. 
The fallacies most likely to be met with are antimony 
and sulphur, both of which give a dark coating to the zinc, 
and evolve a gas which has the power of blackening the 
nitrate of silver ; but these fallacies are completely guarded 
against in the subsequent stages of the process. 
This test acts in water containing _^^^th part of ars 
nic, and it is not difficult to discover the ^ ln °f a g ra iu 
even when mixed with many ounces of organic matter, and 
by careful management it is possible to detect a much 
smaller quantity. 
The principal precautions requisite to be attended to in 
applying this method are, that the solutions be not boiled 
too rapidly, that enough zinc be used to precipitate the 
whole of the arsenic in a thin film, that the suspected fluid 
be never made so acid as to act upon the zinc and liberate 
a gas, that the zinc be very carefully washed before being 
introduced into the hydrogen apparatus, that the reduction 
tube contain no lead, and of course that the zinc and sul- 
phuric acid be pure. — -Chem. Gaz. 
