ARSENIC is a peculiar metal, frequently met with in nature ; sometimes in its pure metallic state, but more commonly combined with other metals, as iron and cobalt, or with sulphur, and frequently united with oxygen. Arsenic has a steel gray colour and consider able brilliancy ; its density is about 51 times that of water. The native metal is granular, and the artificial crystalline; it is extremely brittle, and consequently easily powdered. When arsenic is exposed to the air, it soon loses its lustre, and becomes black on the surface. When kept under water, it under goes no change ; if heated to 356° Fahrenheit, it is volatilized, without previous fusion ; the vapour has a strong smell, resembling that of garlic, and this, to a certain extent, is relied upon as proof of its presence ; the vapour readily condenses in small brilliant crystals of metallic arsenic.
Arsenic and oxygen combine in two propor tions, and both compounds possess acid pro perties ; that which contains the smaller quantity of oxygen is termed arsenious acid, the white arsenic of the shops. Arsenic acid, which contains the larger quantity of oxygen, exists in nature much more commonly than the arsenious acid, in combination with lime, copper, iron, and lead. It is of a milk-white colour, and is very poisonous. Arsenic and hydrogen combine to form a gas, which, how ever, when subjected to intense cold, is con densed into a limpid liquid resembling ether. The gas has an extremely fetid smell. The chloride of Arsenic is a colourless volatile liquid, which combines with oil of olives and of turpentine, and partially dissolves sulphur and phosphorus 'when heated. Arsenic and sulphur may be made to combine in four dif ferent proportions; of which two yield rcalgar and orpiment. Realgar, a red sulphuret, is of a deep-red colour, brittle, easily reduced to powder, inodorous, tasteless, and insoluble in water. Orpiment, a yellow sulphuret, is commonly composed of thin plates, which are of a very fine yellow colour, and flexible to a considerable degree.
Arsenic and metals in general combine with great facility: those which are malleable it renders brittle, and those which are difficult to melt it renders fusible. These alloys do not form an important class of bodies.
The arsenic and arsenious acids combine with many alkalies and metals to produce salts. These, as well as other preparations of arsenic, are largely used in the arts. Arse nite of potash is used in making cobalt blue; the red sulphuret is used in making fire works ; the yellow sulphuret is used as a dye, as a pigment, and in making fire works ; arsenic in its metallic state is em ployed for a few purposes, generally in an alloyed state. The medical applications of arsenial compounds we do not notice here ; further than to say that the common arsenic of the shops is arsenious acid, and that it is too frequently adulterated with plaster of paris and other cheap substances.
The late Mr. Marsh devised some extremely delicate means of detecting the presence of arsenic. The liquid suspected to contain the poison is acidulated with sulphuric acid and placed in contact with metallic zinc, both of which must be previously ascertained to be free from arsenic ; if any arsenic be present in the liquid, the hydrogen gas generated re duces and dissolves it, forming arsenuretted hydrogen gas. This gas is to be lighted at a jet, and a piece of white porcelain or of glass is to be held over the flame, when, if any arse nic be present, a brilliant black spot of metal lic arsenic is deposited on the glass or porce lain. Another method of proceeding is to boil a slip of bright metallic copper in the fluid suspected, previously acidulated with hydrochloric acid. If arsenic be present, the copper is covered with a whitish alloy, and then by heating the metal in a test tube, the arsenic is volatilized, and sublimes in the metallic state in the cold part of the glass. This method is susceptible of detecting very minute portions of arsenic.
Foreign arsenic may be imported into this country duty free.