as contained in _Fowler’s solution_. 5. The arsenite of potass; 6. The
various sulphurets, pure and impure, namely, _realgar_, _orpiment_, and
_king’s yellow_; and 7. Arseniuretted-hydrogen gas.
_Of the Tests for Fly-powder._
This substance is rarely known as a poison in Britain, but is a familiar
poison in France and Germany, under the names of _Poudre à mouches_, and
_Fliegenstein_. Of late it has been occasionally used in Scotland for
poisoning rats.
It is a fine grayish-black powder, formed by exposing powdered arsenic
for a long time to the air; but it also frequently contains fragments of
the metal. It is usually considered by chemists to be a mixture of
metallic arsenic and its white oxide.
It is acted on by water, the white oxide being found ere long in
solution by its proper tests. Oxidation and solution, however, are also
effected upon pure metallic arsenic in the same manner. A thousand
grains of water take up a grain in the course of half an hour when
boiled on the metal.[490]
A very simple and decisive test for fly-powder is derived from the
effect of heat. If it is heated in a tube two substances are sublimed,
first a white crystalline powder, and then a bright metallic crust, the
former being the white oxide, the latter the metal. The metallic crust
thus formed possesses physical properties, which distinguish arsenic
from all other substances, capable of being sublimed by a low heat: The
surface next the tube is very like polished steel, being a little darker
in colour, but equal in brilliancy and polish; and the inner surface is
either brilliantly crystalline to the naked eye, like the fracture of
cast-iron, or has a dull grayish-white colour, but appears crystalline
before a common magnifying lens of four or five powers. If these
characters be attended to, particularly the appearance of the inner
surface, it appears to me scarcely possible to mistake for an arsenical
crust any other substance which can be sublimed by any of the methods
for subliming arsenic.
If a farther test should be desired, it is only necessary, as was first
proposed by Dr. Turner of London,[491] to chase the crust up and down
the tube with the spirit-lamp flame till it is all oxidated, when little
octaedral crystals of adamantine lustre are formed, on which, either
with the naked eye or with the aid of a common lens, triangular facettes
may be distinguished.
The niceties to be attended to in applying the preceding tests will be
considered presently under the head of the next compound, the
sesquioxide.