_Oxide of Zinc._
This is occasionally used internally as a tonic, and may be exhibited in
the form of pill. It is however principally employed externally, as a
mild but efficient astringent; viz. _Ung: Zinci_. ADULTERATIONS. Dr.
Roloff of Magdeburg has lately discovered the casual presence of
_Arsenic_[719] in this oxide; by boiling the substance in distilled
water, and assaying the solution with the ammoniaco-nitrate of silver,
its presence may be instantly recognised; _Chalk_ may be detected by
sulphuric acid exciting an effervescence; and _White Lead_, by its
forming an insoluble sulphate of lead. It ought to be volatile.