594 poisons : their effects and detection. [§ 768. 
The mirror or crust of arsenic is usually described and weighed as 
being composed of the pure metal; but J. W. Rettgers has investigated 
the matter, and the following is an abstract of his results :— 
There is no amorphous form of arsenic, the variety generally so 
called being crystalline. Two modifications can be distinguished : the 
one being a hexagonal silver-white variety possessed of metallic lustre, 
specifically heavier and less volatile than the second kind, which is 
black in colour, crystallises apparently in the regular system, and 
constitutes the true arsenic mirror. The former modification corresponds 
to red hexagonal phosphorus (red phosphorus having been recently 
proved by the author to be crystalline), and the latter to yellow 
phosphorus, which crystallises in the regular system. Both modifica¬ 
tions of arsenic are perfectly opaque; deposits which are yellow 
or brown, and more or less transparent, consist of the suboxide and 
hydride, As 2 0 and AsH. The brown spot on porcelain produced by 
contact with a flame of arseniuretted hydrogen is not a thin film of 
As, but one of the brown solid hydride AsH, formed by the decomposi¬ 
tion of AsH 3 . This view is confirmed by the fact that arsenic sublimed 
in an indifferent gas (e.g. C0 2 ) is deposited in one or other of the 
modifications described above, the brown transparent product being 
obtained only in the presence of H or 0. Moreover, pure arsenic is 
insoluble in all solvents, whereas the film on porcelain (AsH) is soluble 
in many solvents, including hydrocarbons of the benzene series (e.g. 
xylene), warm methylene iodide, and hot caustic potash. 
Hence quantitative results from weighing arsenical mirrors can 
never be ‘accurate, because the mirrors consist of mixtures of hydride 
and suboxide. 
Reinsch’s Test. A piece of bright copper foil, boiled in an acid 
liquid containing either arsenic or antimony, or both, becomes coated 
with a dark deposit of antimony or arsenic, as the case may be. The 
arsenical stain, according to Lippert, is a true alloy, consisting of 1 
arsenic to 5 copper. 1 Properly applied, the copper will withdraw every 
trace of arsenic or antimony from a solution. 
Copper gauze or copper foil is oxidised in the air by heating in an 
open tube to a gentle red heat. The film of black oxide is next dissolved 
off by a few seconds’ immersion in strong nitric acid, leaving a bright 
chemically clean surface. The acid is removed from the copper by wash¬ 
ing in a stream of water. A piece of copper thus prepared, about 1 inch 
X j inch, is suspended by means of a thin platinum wire in from 50 to 100 
c.c. of the liquid to be examined, the liquid acidified by HC1 and gently 
boiled for twenty minutes. Operating on organic liquids, the copper 
is usually darkened in colour, even if arsenic-free. Any black stain 
may be caused by sulphur, by organic matter, by arsenic, or by antimony. 
1 Journ. f. prakt. Chem ., xiii. 168. 
