This is obtained in roasting the ores of other metals containing arsenic. it is condensed in the flues, collected, and refined by sublimation.

Properties

As first procured, it is in beautifully transparent, glasslike, colourless or slightly yellowish masses, which, however, gradually become white and opaque, so as to resemble white porcelain or enamel; the change commencing at the surface, and gradually penetrating towards the centre. This change is supposed by some to be merely molecular, by others has been ascribed, with greater probability, to the absorption of a minute proportion of water. in the shops, it is either in the state of fragments, which have undergone the change more or less completely, or of a fine, perfectly white, rather heavy powder. its specific gravity is stated variously at from 3.2 to nearly 3.8. it is quite inodorous, even in vapour, and has a feeble, slightly sweetish taste. its solubility varies with its transparency, being, according to Bussy, three times as great in the perfectly transparent as in the perfectly opaque variety. Cold water dissolves but little of either, not more than one or two parts in one hundred; boiling water dissolves a much larger proportion, and, upon cooling, the solution retains considerably more than cold water will take up. The presence of organic matters diminishes the solvent power of water. The acid is soluble in alcohol and the oils. it is fusible and wholly vaporizable by heat; and, though the vapour of the acid is inodorous, yet, if the vaporization be effected in the presence of combustible matter, as by throwing the powder upon red-hot coals, a strong odour of garlic is exhaled, owing to the deoxidation of the acid; the odorous vapour being that of the reduced metal. Arsenious acid consists of one equivalent of arsenic and three of oxygen.

* The late Dr. Nelligan, of Dublin, on a visit I made to Dublin some years since, informed me that he had found great advantage from changing the arsenical preparations in the treatment of skin diseases; having been able to effect a cure at one time by the use of a preparation which had failed to cure the same affection at another time, and, after having tried one of the preparations without effect, resorting to another with the most satisfactory results. [Note to the third edition.)

Incompatibles. With solution of arsenious acid, lime-water throws down a white precipitate of arsenite of lime; hydrosulphuric acid a whitish, and soluble sulphurets a yellow precipitate of sulphuret of arsenic; ammonio-sulphate of copper, a green precipitate of arsenite of copper; and ammonio-nitrate of silver, a yellow precipitate of arsenite of silver. in relation to the means of detecting arsenious acid for medico-legal purposes, the reader is referred to works on chemistry and toxicology. He will find them accurately stated by the late Dr. Bache in the U. S. Dispensatory.

The effects of arsenious acid on the system, and its other medicinal relations have been already sufficiently considered. The ordinary dose is one-twelfth of a grain twice or three times daily, which may be diminished or increased according to circumstances before stated. it may be given in pill, made with the crumb of bread; very great care being taken to diffuse the arsenical preparation equably through the mass, for which purpose it should be well rubbed in a mortar with powdered gum arabic or sugar, before being incorporated with the crumb. A safer method, however, of administering the medicine is in solution, especially when it is to be continued long; as in this state it will be less liable to accumulate, and thus at any time to operate with unexpected violence. The most convenient solution is the officinal one to be noticed immediately. Should the medicine irritate the stomach or bowels, it may sometimes be advantageously combined with a little opium; but, as a general rule, the occurrence of such irritation should be a signal for diminishing the dose, or suspending the use of the remedy; as it appears to be a generally admitted fact, that the arsenical preparations act most favourably in chronic disease, when given in such quantities as to produce none of the symptoms characteristic of their poisonous action. Arsenious acid is best administered when there is some food in the stomach, as it may thus be slowly absorbed into the system, with less opportunity of irritating contact with the gastric mucous membrane.