_Fatal Dose._--One grain and a half.
_Fatal Period._--Four hours; more commonly two to four days.
_Detection of Phosphorus in Organic Mixtures._--Mitscherlich's method is
the best. Introduce the suspected material into a retort. Acidulate with
sulphuric acid to fix any ammonia present. Distil in the dark, through
a glass tube kept cool by a stream of water. As the vapour passes over
and condenses, a flash of light is perceived, which is the test.
XXI.--ARSENIC AND ITS PREPARATIONS
=Arsenic= is the most important of all the metallic poisons. It is much
used in medicine and the arts. It occurs as metallic arsenic, which is
of a steel-grey colour, brittle, and gives off a garlic-like odour when
heated; as arsenious acid; in the form of two sulphides--the red
sulphide, or realgar, and the yellow sulphide, or orpiment; and as
arsenite of copper, or Scheele's green. It also exists as an impurity in
the ores of several metals--iron, copper, silver, tin, zinc, nickel, and
cobalt. Sulphuric acid is frequently impregnated with arsenic from the
iron pyrites used in preparing the acid. It is a constituent of many rat
pastes, vermin or weed killers, complexion powders, sheep dips, etc.
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