_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.
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