The detection of hydrocyanic acid requires special precautions. The substances to be examined are mixed with water, if solids are present, and introduced into a retort provided with a delivery-tube which dips in a solution of nitrate of silver. The retort is then heated over a water-bath. If the evolved vapors produce a precipitate in the silver solution, the heating is continued until a fresh portion of the latter is no longer affected. The operation is now interrupted, hydrochloric acid added to the retort, and heat again applied. Should a second precipitation of cyanide of silver occur, the presence of a _cyanide_ in the suspected materials is indicated; whereas the formation of a precipitate by the simple action of heat would point to the presence of free hydrocyanic acid or cyanide of ammonium.[H] In case the latter compound is present, ammonia will be contained in the distillate. [H] Ferrocyanides and ferricyanides--non-poisonous compounds--likewise, evolve hydrocyanic acid when distilled with a strong acid. Their presence is indicated by stirring a small portion of the materials with water, filtering the fluid, acidulating the filtrate with hydrochloric acid, and testing two portions: one with sesquichloride of iron, the other with protosulphate of iron. If either of the above salts be present, a blue precipitate is produced.--_Trans._ In order to identify the cyanogen, a portion of the precipitate is collected upon a small filter, washed, dried, and then allowed to fall into a rather long tube, closed at one end, in the bottom of which some iodine has previously been placed. A column of carbonate of soda is then introduced above the precipitate for the purpose of retaining the excess of iodine probably taken. Upon heating the lower end of the tube, white fumes of iodide of cyanogen, which condense in needles upon the cold portion of the tube, are produced. These are easily recognized by aid of a magnifying glass. They are colorless and are readily volatilized by heat. Some ammonia is next added to a solution of protosulphate of iron, the precipitate formed thoroughly washed, and exposed to the air until it acquires a greenish hue. The iodide of cyanogen is then withdrawn from the tube and mixed with potassa-lye and the precipitate mentioned above. The mixture is evaporated to dryness, the residue obtained treated with water and the filtered solution then acidulated with hydrochloric acid. If a solution of a persalt of iron is now added to the fluid, a blue precipitate is formed. The addition of salts of copper produces a reddish precipitation. The remainder of the precipitate formed in the nitrate of silver solution is heated with sulphur and then boiled with an aqueous solution of chloride of sodium: if cyanogen is contained in the precipitate, a solution of sulphocyanate of soda will be formed, and upon adding sesquichloride of iron an intense red coloration produced.