ters: In the centre a deposit of metallic arsenic, round this a
mixture of metallic arsenic and arsenious acid, and outside this another
ring of arsenious acid in octahedral crystals. The deposit is dissolved
by a solution of chloride of lime, turned yellow by sulphide of ammonium
after evaporation; on the addition of strong nitric acid, evaporated and
neutralized with ammonia and nitrate of silver added, a brick-red colour
is produced--arseniate of silver.

_Reinsch's Process._--Boil distilled water with one-sixth or one-eighth
of hydrochloric acid, and introduce a slip of bright copper. If, after a
quarter of an hour's boiling, there is no stain on the copper, add the
suspected liquid. If arsenic be present, it will form an iron-grey
deposit. If this foil be dried, cut up, put in a reduction-tube, and
heated, crystals of arsenious trioxide will be deposited on the cold
part of the tube.

These tests are difficult to apply, but as arsenic is a ubiquitous
poison, and as there are many sources of fallacy, it would be well, when
possible, to obtain the services of an expert.

_Biological Test._--Put the substance to be tested into a flask with
some small pieces of bread, sterilize for half an hour at 120 deg. C. When
cold, inoculate with a culture of _Penicillium brevicaule_, and keep at
a temperature of 37 deg. C. If arsenic is present, a garlic-like odour is
noticed in twenty four hours, due to arseniuretted hydrogen or an
organic combination of arsenic. This test is delicate, and will detect
1/1000 of a milligramme, but it is not quantitative.

=Other Preparations of Arsenic.=--These are arsenite of potash (Fowler's
solution), cacodylate of sodium, and arsenite of copper (Scheele's
green), the last frequently used for colouring dresses and wall-papers.
Persons using these preparations may suffer from catarrhal symptoms,
rashes on the neck, ears, and face, thirst, nausea, pain in stomach,
vomiting, headache, perhaps peripheral neuritis and loss of patellar
reflex. The cacodylates, although formerly employed in the treatment of
phthisis, should be used with the utmost caution. The arsenites give the
reactions of arsenious acid.

Arsenic is eliminated not only by the kidneys and bowels, but by the
skin, and in women by the menses. It may be detected in the sweat, the
saliva, the bronchial secretion, and, during lactation, in the milk.

The sale of arsenic and its preparations to the public is properly
hedged round w