Selcnic acid is liquid, colourless, inodorous, of an oleaginous consist ence, and very acid; it attracts moisture from the air, and always retains about per cent. of water, which cannot be expelled with out decomposing the acid. It may be heated to 536' without decom position, but at 554' it is rapidly converted into ox.iysen and selanious acid. When concentrated at a temperature of 3297, its sp. gr. is 2.524, and It gradually Increases up to the temperature of 545', when it becomes 2-625. When mixed with water, much heat is evolved. Its salts are called Sdeniates. Zinc and iron are dissolved, by this acid with the evolution of hydrogen gas, and copper with the formation of selenious acid.
&kaiak of soda (NaO, Se0, + 10 aq.) occurs in large crystals like those of Glauber's salt. Seleniales of time (CaO, Se0, + 2aq.) and of nickel ( NiO, Se0,-i- 6aq. ) resembles the corresponding sulphates. &leniate of Oedmiuns (0.10,Se0,4- 2aq) differs both in composition and crystalline form from the sulphate of cadmium, the crystals also lone an equivalent of water on being heated to 212' Fehr.
Hydroyen and Selenium (HSe) unite to form hydroselenic acid or seleniuretterl hydrogen. It Is easily obtained by the action of diluted sulphuric, acid upon selenide of potassium, or of iron. This gas is colourless, has a disagreeable odour, and so powerfully irritates the membrane lining the nose as to excite symptoms of catarrh, and destroy the sense of smelling for Home hours. Water readily dissolves this gas : the solution is at first colourless, but after a time it acquires a reddish hue ; it gives a brown stain to the skin, and reddens litmus paper. When exposed to the atmosphere it is decomposed, its hydro gen being absorbed by oxygen, and the selenium is deposited. It
decomposes the solutions of many metallic salts, selenides of the metals being precipitated.
Chlorine and Selenium combine when the gas is passed over the selenium; heat is evolved, and a brown chloride is obtained, which is liquid ; it is heavier than water, not very volatile, and by the action of water is eventually decomposed, and resolved into selenious and hydrochloric acids and selenium.
This dichloride cSe„Cl) may be converted into a bichloride by exposure to excess of chlorine. It ie a white solid compound which is volati lised when heated, and condenses in small crystals. It is decom posed by water.
Sulphur and Selenium (S,Se). A definite compound of these is formed by passing htirosulphurie acid gas into a solution of hydro selenic acid ; the fluid assumes a yellow colour, becomes turbid, and a precipitate is formed, though but slowly, unless a little hydrochloric acid be added. When exposed to heat this disulphide of selenium becomes red and viscid, and at high temperatures may be distilled without decomposing.
Phosphorus and Selenium combine when the selenium is dropped into melted phosphorus ; the product is a red substance, but it does not appear that any definite compound of these bodies is formed.
Selenethyl or selenium-ethyl. [ETHYL, seknide of ethyl.] Selenium forms sdenides with most of the metals, and the selenida of lead ie one of the most abundant of the native compounds which occur in the Hartz. [Lean, in NAT. HIST. Div.) The characteristic properties of selenium are those of tinging the flame of the blowpipe of a light blue colour, and emitting an acrid vapour, when heated in the air, which has the peculiar smell of decayed horse-radish.