Arsenic (Gr. or male, from its power in destroying), the common name of the white oxide of arsenic, or arseni-ous acid. In ancient times the name was applied to a reddish-colored mineral compound of arsenic and sulphur, a substance in use then as a medicine, and also in painting. Metallic arsenic occurs native in veins in the crystallized rocks and older slates, and it is also prepared by subliming its oxide in presence of a reducing flux, and protected from the air. Many modern chemists do not regard it as a metal, though it is commonly treated as such. Combined with oxygen, it unites with metals, forming arsenites and arseniates of these metals, but is never itself the base of any salt. The ores of the metal are not therefore carbonates and sulphates of its oxide, but combinations of the metal itself with sulphur, forming the sul-phuret, and this combined with iron, cobalt, or nickel; or they are oxides of the metal, or else compounds of its oxides with other metals. It is remarkable as the most volatile and one of the most combustible of the metals, is readily sublimed at a temperature of 860° F., apparently before it melts, and at a greater heat it takes fire and burns with a pale blue flame.
In subliming, it gives out dense fumes of a peculiar garlicky odor, which distinguish it from other substances even when present in very minute quantity. It is more brittle than antimony, and may be reduced to fine powder in a mortar. Freshly prepared, it has a brilliant metallic appearance, a bluish-white color, and crystalline structure; but in the air the metal becomes black and crumbles to powder. In water it may be kept without change. Its specific gravity is 5.96. It is the softest of the solid metals, its hardness being rated on the mineralogical scale at 3.5. Arsenic readily combines as an alloy with other metals, rendering them more fusible and brittle. Its presence is particularly injurious in iron ores, making the cast metal exceedingly brittle; but it gives great fluidity to the melted iron, so that for tine castings that do not require much strength, but sharply defined and delicate outline, it is sometimes desirable. It also increases the brightness of some alloys. It is not employed for any useful purposes in the metallic state. - Arsenious acid, or white arsenic, is the most common combination of this metal. It is the sublimate, which escapes when arsenic is heated in the open air.
The metal combines in the proportion of 1 equivalent with 3 of oxygen, the compound consisting of arsenic 75.76 per cent. and oxygen 24.24 per cent. The sublimate, after exposure, is a white powder, but may be collected in the form of a glassy, transparent cake, or crystallized in octahedrons. It is partially soluble in boiling water, and less so in cold water. The solution is slightly acid, having but a feeble reaction upon litmus paper. The following are some of the most important tests given for detecting the presence of this poison: The blowpipe develops its peculiar odor, with little liability of mistake, in arsenical matters, heated on charcoal. It also reduces the metal, and causes it to condense in the form of a metallic ring in the cold part of a glass tube, in which the substance containing arsenious acid has been placed with carbonate of soda and charcoal, and heated. The presence of arsenic may be shown by this method, when the particle containing it is so small as to be invisible to the naked eye, in the following manner, communicated by Prof. A. K. Eaton of New York: The microscopic particle is placed in a bulb of a small glass tube, and a fine splinter of charcoal is placed by the side of it. The whole should then be thoroughly dried.
The neck of the bulb is next to be drawn out to a capillary tube, and cooled. On applying heat to the matter in the bulb, this produces by sublimation a plainly visible arsenical ring in the fine bore of the tube. The acid is precipitated from its solutions by sulphuretted hydrogen in the form of tersulphuret of arsenic of a lemon-yellow color. This is a very accurate test, and is so delicate that the yellow tint is apparent when only 1/10,000 of the acid is present, and the precipitate when the arsenious acid is in the proportion of 1 part to 80,-000 of water. It is precipitated in a white powder by excess of lime water, when forming 1/500 part of the liquid. Ammonio-sulphate of copper gives an apple-green precipitate, apparent when the acid forms 1/12,000 part. A still more delicate test is that of Prof. Reinsch, to place a slip of bright copper leaf in the aqueous solution acidulated with hydrochloric acid; a gray film of arsenic is deposited upon the copper, showing the presence of less than 1/100,000 part of the acid. It is affirmed that even 1/250,000 part of arsenic will not escape detection by this test. Nitrate of silver gives with it a yellow precipitate.
It should be borne in mind, in attempting to determine the presence or absence of arsenious acid in any mixture in which organic substances, particularly those which are not volatile, are present, that some of these substances often produce very similar reactions, and, on the other hand, that they prevent or modify those which arsenious acid should produce in mixtures where no organic substances are present. - "Marsh's apparatus "has been long known as affording an easy means of detecting the presence of arsenious acid. The process depends on the property possessed by arsenic of forming a gas with hydrogen, and depositing itself in the metallic state upon the surface of a cold plate held over the flame of the burning gas. Hydrogen is prepared in the usual way, with granulated zinc and diluted sulphuric acid, in a glass flask provided with a tube of glass drawn out to a small orifice at its outer end; or a mere tube itself may be used, bent in the form of the letter U, one end drawn out, the other left open for introducing the materials, and closed with the thumb when in use. The hydrogen evolved should first be tested by burning it against a porcelain plate to prove that it is free from arsenic, and then the suspected liquid is to be introduced into the apparatus.