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BENZOIC ACID. 207

delicate test, however, is that of nascent hydrogen, known as
Marsh's Test, which consists in subjecting the arsenic to the
action of nascent hydrogen (evolved by the action of diluted sul-
phuric acid on pure zinc) ; it is deoxidized, and unites with the
hydrogen to form arseniuretted hydrogen gas, which has the
odor of garlic, and burns with a bluish-white flame, depositing a
black spot of metallic arsenic on the surface of a cold plate held
directly in the jet.
Reinsch's Test consists in boiling the suspected material with
hydrochloric acid and clean copper foil, when, if any arsenic is
present, the copper foil becomes coated with gray metallic
arsenic.
Cobalt.— Cohaltum [Formula^ Co)— is a metal chiefly found in
combination with arsenic, either in the form of the arsenide {tin-
white cobalt)^ or as gray cobalt ore, with sulphur and arsenic.
The late Dr. Robert Arthur, at one time, preferred cobalt as a
devitalizing agent to arsenious acid, being of the opinion that less
irritation followed its action, and consequently greater immunity
from peridental inflammation ; afterwards, however, he found it
less reliable than arsenious acid. At the present time cobalt is
not considered to be any safer than arsenic; it is also less prompt
in its action as a devitalizing agent.

ACIDUM BENZOICUM—BENZOIC ACID.

Formula.—Q.Y{f^.r^Q^.—QO,OY{.
Derivation.—Benzoic acid is obtained from benzoin (a bal-
samic resin, which exudes from the incised stem of a tree of
Sumatra, Java, Borneo and Siam), either by sublimation or by the
action of alkalies ; it is also made from hippuric acid. It is in
the form of white, feathery crystals, of a silky lustre, a peculiar,
agreeable odor, and warm, acidulous taste. While it is sparingly
soluble in cold water, it is more soluble in warm or boiling water,
and very soluble in alcohol, solutions of potassa, soda, ammonia,
lime and concentrated sulphuric and nitric acids. The fixed oils
also dissolve it. fVom solution it crystallizes in transparent
prisms. It is also inflammable.