OF POISONING WITH ARSENIC.
The third order of the irritant class of poisons includes the compounds
of the metals. These are of great importance to the medical jurist. They
are frequently used for criminal purposes; they give rise to the
greatest variety of symptoms; and the medical evidence on trials
respecting them, while much skill is required on the part of the witness
to collect it, is also the most conclusive.
It must not be inferred from their being arranged in the class of
irritants that their action is merely local. In fact this is the case
with a very few of them only, which produce chemical corrosion. The
greater number likewise act indirectly on organs at a distance from the
part to which they are applied. Nevertheless the most prominent symptoms
generally produced by them are those of violent local irritation; so
that they may be justly considered in the place which has been assigned
them.
The poisons included in this order are the oxides and salts of arsenic,
mercury, copper, antimony, tin, silver, gold, bismuth, iron, chrome,
zinc, barium, lead. Many other metals also form poisonous compounds with
various acids and other bodies; but these are so rare as to be merely
objects of physiological curiosity.
Of all the varieties of death by poison, none is so important to the
medical jurist as poisoning with arsenic. On account of the shameful
facility with which it may be procured in this country, even by the
lowest of the vulgar, and the ease with which it may be secretly
administered, it is the poison most frequently chosen for the purpose of
committing both suicide and murder. In 1837 and 1838 no fewer than 186
cases of fatal poisoning with arsenic were known to have occurred in
England alone (see p. 90). Of 221 cases of murder by poison in France
during ten years subsequent to 1829, in which the poison given was
ascertained, there were 149 where the substance administered was
arsenic.[487] It is fortunate, therefore, that there are few substances
in nature, and perhaps hardly any other poison, whose presence can be
detected in such minute quantities and with so great certainty.
SECTION I.—_Of the Chemical Tests for the Compounds of Arsenic._
Metallic arsenic has an iron-gray colour, a specific gravity of 8·308,
and a crystalline fracture. It is very brittle. It has a strong tendency
to oxidate, so that it undergoes this change in air, in water, and even
in alcohol. In air, particularly when moist, it becomes rapidly
tarnished, a black powder being formed, which some have regarded as a
regular protoxide.[488]—When exposed to heat, metallic arsenic is
usually said to sublime at the temperature of 356° F.; but according to
some late experiments by Dr. Mitchell of Philadelphia this does not
happen under a low red heat, luminous in the dark.[489] In close vessels
it condenses unchanged; but when heated in the open air, it passes to
the state of white oxide, and rises in white fumes. This substance is a
sesquioxide, consisting of two equivalents of metal and three of oxygen.
Another oxide likewise exists, which contains two equivalents of metal
and five of oxygen, and, possessing strong acid properties, is
denominated arsenic acid. The sesquioxide and arsenic acid unite with
bases, and produce compounds which, with the exception of those they
form with the alkalis, are mostly insoluble. Metallic arsenic unites
with sulphur in two proportions, forming an orange-red and a
sulphur-yellow compound. The compounds of arsenic have very little
chemical action with vegetable and animal principles.
Of the compounds which arsenic thus forms, those which it will be
necessary to particularize are the following:—1. The protoxide of
Berzelius, or _fly-powder_. 2. The arsenious acid, or _white arsenic_.