PRESENCE OF ARSENIC. 611 
seen any nitric acid produced through the agency 
of the impure sulphuric; but, probably, in such, 
arsenic would likewise be found. 
Before using any one of the three acids, we 
cannot be too careful in minutely examining its 
purity. 
In Mr. Marsh’s communication to the Society 
of Arts, he suggests determining the purity of 
the zinc by putting a bit of it into the apparatus, 
with some diluted sulphuric acid only; and if the 
gas obtained on being set fire to, as it issues from 
the jet, will deposit no metallic film on a bit of 
flat glass submitted to the flame, and yield no 
white sublimate within an open tube; the zinc 
may be regarded as in a fit state for use. The 
purity of the zinc being known, the like process 
might, of course, be resorted to in determining 
the fitness of the sulphuric acid for use ; but, I 
will remark as a caution, that in determining the 
freedom of either the zine, or the acid, or the ap- 
paratus from arsenic, it is much preferable to hold 
a cold substance of large bulk to the flame, froma 
small jet, instead of a bit of glass, for I have found 
that when the gas contains only an exceedingly mi- 
nute quantity of arsenic, a distinct metallic deposit 
