Page:Treatise on poisons in relation to medical jurisprudence, physiology, and the practice of physic (IA treatiseonpoison00chriuoft).pdf/204

 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. 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.

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. —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. In close vessels it condenses unchanged; but when heated in the open air, it passes to the state of white oxide, and rises in whiteBuchner once made a similar observation, and is inclined to think that oxidation does not occur, if the metal is quite pure. [Repertorium für die Pharmacie, xxi. 29.]]