_Nitric Oxyd of Mercury—Red Precipitate._
QUALITIES. _Form_, small scales of a bright red colour; _Taste_, acrid
and corrosive. CHEMICAL COMPOSITION. It is strictly speaking a
_sub-nitrate_ of mercury, for if it be boiled for a short time with six
times its weight of water, the liquor when filtered yields a precipitate
with ammonia. SOLUBILITY. It is slightly soluble in water, but
extensively in nitric acid, without any effervescence. USES. It is used
only externally, as an escharotic. OFFICINAL PREP. _Unguent._
_Hydrargyri Nitrico-oxyd._ L.E.D. ADULTERATIONS. _Red Lead_ may be
detected by digesting it in acetic acid, and adding sulphuret of
ammonia, which will produce a dark-coloured precipitate: it should be
totally volatilized by heat.