Treatise on Poisons by Sir Robert Christison

as contained in _Fowler’s solution_. 5. The arsenite of potass; 6. The various sulphurets, pure and impure, namely, _realgar_, _orpiment_, and _king’s yellow_; and 7. Arseniuretted-hydrogen gas. _Of the Tests for Fly-powder._ This substance is rarely known as a poison in Britain, but is a familiar poison in France and Germany, under the names of _Poudre à mouches_, and _Fliegenstein_. Of late it has been occasionally used in Scotland for poisoning rats. It is a fine grayish-black powder, formed by exposing powdered arsenic for a long time to the air; but it also frequently contains fragments of the metal. It is usually considered by chemists to be a mixture of metallic arsenic and its white oxide. It is acted on by water, the white oxide being found ere long in solution by its proper tests. Oxidation and solution, however, are also effected upon pure metallic arsenic in the same manner. A thousand grains of water take up a grain in the course of half an hour when boiled on the metal.[490] A very simple and decisive test for fly-powder is derived from the effect of heat. If it is heated in a tube two substances are sublimed, first a white crystalline powder, and then a bright metallic crust, the former being the white oxide, the latter the metal. The metallic crust thus formed possesses physical properties, which distinguish arsenic from all other substances, capable of being sublimed by a low heat: The surface next the tube is very like polished steel, being a little darker in colour, but equal in brilliancy and polish; and the inner surface is either brilliantly crystalline to the naked eye, like the fracture of cast-iron, or has a dull grayish-white colour, but appears crystalline before a common magnifying lens of four or five powers. If these characters be attended to, particularly the appearance of the inner surface, it appears to me scarcely possible to mistake for an arsenical crust any other substance which can be sublimed by any of the methods for subliming arsenic. If a farther test should be desired, it is only necessary, as was first proposed by Dr. Turner of London,[491] to chase the crust up and down the tube with the spirit-lamp flame till it is all oxidated, when little octaedral crystals of adamantine lustre are formed, on which, either with the naked eye or with the aid of a common lens, triangular facettes may be distinguished. The niceties to be attended to in applying the preceding tests will be considered presently under the head of the next compound, the sesquioxide.