ARSENIOUS ACID.

381

experienced in removing it from the filter for reduction. The readiestway is that recommended by Devergie:—Collect it on the fdter in assmall a space as possible, then wash it with liquor ammonia;, which dis-solves it. The filtered liquid may then be evaporated in a capsule orwatch-glass : the ammonia flies off, and leaves the sesquisulphuret.

'Yh& fallacies of the liydrosulphuric acid test are, the salts of cadmium,the persalts of tin, the antimonial compounds, and selenic acid, whichoccasion precipitates with liydrosulphuric acid, more or less analogous incolour to that produced by arsenious acid. The precipitate with cadmiumclosely resembles that with arsenic. This metal (cadmium) has beendetected in some of the preparations of zinc ( vide Thomson’s Hist, ofChemistry, ii. 220). The perchloride of tin, sold for the use of dyersunder the name of spirit of tin, occasions a yellow precipitate (bisul-phuret of tin) somewhat resembling sesquisulphuret of arsenicum. Veryweak solutions of emetic tartar form a reddish yellow liquid, or throwdown a reddish precipitate (sesquisulphuret of antimony) somewhat ana-logous in appearance to that formed by an arsenical liquid. If hydrosul-phuric acid be transmitted through a liquid in which pulvis antimo-nialis has been boiled, the solution acquires a yellowish red colour, fromthe formation of some sesqui- or bisulphuret of antimony. From all theabove precipitates sesquisulphuret of arsenicum is readily distinguishedby the reduction test already mentioned.

Ilydrosulphate of ammonia (described at p. 271) is sometimes em-ployed as a substitute for liydrosulphuric acid, an acid being added atthe time of applying it, to neutralize the ammonia; but it is liable toseveral serious objections. When fresh prepared, it causes a yellowishprecipitate with arsenious acid, red with emetic tartar, and black with solu-tions of lead; but by exposure to the air for a day or two it forms a whiteprecipitate with arsenious acid, yellow with emetic tartar, and red with lead!

5. Nascent Hydrogen: Marsh's test. —If arsenious acid be submittedto the action of nascent hydrogen, it is deoxidized, and the metallicarsenicum thereby obtained, combining with hydrogen, forms arse-niuretted hydrogen gas.

This test, which is the discovery of Mr. Marsh, of Woolwich {Trans,of the Soc. of Arts, li. 66; also Lond. Med. Gaz. xviii. 650), may be thusapplied:—Mix a small portion of the suspected liquid with some dilutedsulphuric acid (1 oil of vitriol and 7 water), and pour the mixture oversome pieces of zinc previously introduced into a proper apparatus: bub-bles of gas immediately make their appearance. If no arsenious acid bepresent, the evolved gas is hydrogen ; but if the liquor hold arsenic insolution, arseniuretted hydrogen gas is formed. This gas is recognisedby the following characters: it has an alliaceous odour, and burns in theair with a bluish white flame, and the deposition of black metallicarsenicum and white arsenious acid. If a plate of mica, or of commonwindow-glass, or of porcelain (as a white saucer), be held a short distanceabove the flame, arsenious acid, in a finely pulverent state, is depositedon it: if the plate be depressed so as to touch the flame, and therebyslightly to impede the combustion of the gas, a blackish deposit ofmetallic arsenicum is obtained. Or both these deposits may be readilyprocured by holding vertically over the flame a tube of glass, 9 or 10inches long, and a quarter or half an inch in diameter: the tube becomeslined for the space of several inches with metallic arsenicum and