ARSENIOUS ACID is the arsenical compound most familiarly known. It is obtained principally during the roasting of the arsenical nickel ores in Germany in furnaces com municating with flues. When the arsenic of the ore burns, it passes.into the condition of A. A. and rising as vapor into the somewhat cool flue, is there deposited as a grayish powder, known by the names of smelting-house smoke, flowers of arsenic, poison flour, or rough. A. A. In this condition, the A. A. is contaminated with sonic impurities, from which it may be separated by introducing the gray powder into an egg-shaped vessel, and applying heat at the lower end, when the A. A. rises in vapor, and condenses in the cool end as a transparent glassy or vitreous substance. Ordinary A. A. of the shops (which is what is popularly known as arsenic) is a white crystalline powder, which. feels decidedly gritty, like fine sand, when placed between the teeth, and has no well marked taste. It is very heavy, so much so as at once to be noticeable when a paper or bottle containing it is lifted by the hand. It is soluble in water to the extent of 1 part of A. A. in about 100 parts of cold water, and 1 part of A. A. iu about 10 parts of boiling water. As ordinarily sold in quantities under 10 lbs. in weight, the A. A. is required by law to be colored with of its weight of indigo, or of its weight of soot; the object of the admixture being to render any liquid to which the A. A. might be added, with a murderous intent, of a black or bluish-black hue, and thus indicate the presence of something unusual. In packages of 10 lbs. and upwards, A. A. is allowed to be sold in the pure white crystalline form without coloration. When placed in a spoon, or other vessel, and heated, the A. A. volatilizes, and condenses in crystals on any cool vessel held above. By this means, it can be distinguished from ordinary flour, which, when heated, would char, and leave a coal behind; and from chalk, stucco, baking-soda, tooth-powder, and other white substances, which, when heated, remain iu the vessel as a non-volatile white residue. Again, when A. A. is placed on a red-hat cinder, and the escaping vapors cautiously brought under the nostrils, the strong alliaccous odor characteristic of arsenic is given off. The mode in which A. A. comports itself, when thrown upon water, is likewise peculiar. Instead of at once descending through the water like sand, the A. A., notwithstanding its great density (sp. gr. 8.70), partially floats on the surface, as wheat-flour does; and that portion which sinks in the water, rolls itself into little round pellets, which are wetted only on the out side, and contain much dry A. A. within. The solution of A. A. in water is recognized by three tests: 1. Hydrosulphuric acid and hydrochloric acid produce a yellow precipitate of sulphuret f arsenic (AsS3), soluble in ammonia.
2. Ammonio-sulphate of copper, an apple-given, precipitate of arsenite of copper 3. Ammonio-nitrate of silver, a yellow precipitate of arsenite of silver In many cases A. A. is used as a means of destroying animal life, but happily, the processes for the detection of the poison in organic mixtures and in the animal tissues are so unerring and trustworthy, that it is hardly within the range of possibility that an animal can be destroyed by the administration of A. A. without very decided evidence of the existence of the poison being obtained on examination of the various parts of the animal structure; indeed, it may be safely said that there is no limit to the detection of the poison, as even after the animal structure has been so far decomposed that little remains, yet still the poison, from its indestructibility, survives, and will indicate itself Nearly, 6u the application of the several tests.
For the isolation and recognition of A. A. in organic mixtures, such as the contents of a stomach, three processes may be followed. The method generally pursued, and that upon which greatest dependence is placed, is called Reinsch's process, from tke name of its discoverer. The manner of its application is to treat the organic mixture with water sufficient to render it thin, then add hydrochloric acid to the extent of one eighth of the volume of the liquid; apply heat, and when the whole has been raised to near the boiling-point, introduce clean, newly burnished pieces of copper in the form of wire, gauze, or foil. If A. A. be present in the mixture, a steel-gray coating of metallic arsenic will form on the surface of the copper. This apparent tarnishing of the copper may take place when no A. A. is in the mixture, and may be produced by salts of mercury, antimony, etc., as well as by sulphur compounds, and even occasionally by fatty matters. To distinguish between the coating formed by A. A. and that prodUced by other sub stances, the copper is taken out of the mixture, washed with water, to remove acid; immersed in ether, to dissolve off any adherent fatty matter; dried between folds of blotting-paper; introduced into the lower end of a dry glass test-tube, and there cautiously heated. The metallic arsenic (As) is driven off by the heat from the surface of the copper, rises in vapor into the upper portions of the test-tube; there meets the oxygen of the air, with which it combines, forming A. A. (As00, and thereafter deposits itself on the,inner surface of the cool part of the tube in little glistening crystals. On allowing the tube to cool, adding water thereto, and applying heat, the water dissolves the crystals of A. A., yielding a solution, to separate portions of which the liquid tests mentioned above may be successfully applied. This process may likewise be employed in the detection of A. A. in animal tissue, as in the liver, spleen, kidneys, etc., by first dividing the animal matter into small pieces, and thereafter treating with water, hydrochloric acid, and copper. The precautions which require to be exercised in trying this process are, that the hydrochloric acid and copper are themselves free from A. A. Hydrochloric acid has long been known to be liable to contain at times a veryaensible proportion of the poison, and it is therefore neceisary, i sary, before using the acid in auy experiment, to make a pre liminary trial with dilute hydrochloric acid, into which, when heated, a piece of copper is immersed; and if no tarnishing occurs after a quarter of an hour's trial, the acid may be declared free from contamination with arsenical compounds. The liability of copper to contain arsenic some years ago (Aug., 1859) assumed importance in connection with a trial for murder by slow poisoning with arsenic, which took place in Britain. In this case, a considerable amount of copper was dissolved during the.testing, and supplied the poison in quantity enough to produce a faint coating on a piece of copper which was subsequently introduced into the liquid. The result was that A.A. was at first declared to be present in the material under examination; but further experiments demonstrated that the copper itself had afforded the arsenic. To free copper from any arsenic which it may contain originally, it is only necessary to heat the copper over a gas or spirit-lamp flame, when the arsenic volatilizes, and leaves the copper uncontaminated therewith.