ARSENIOUS ACID.

379

well. None of these, however, present any advantage over charcoal savethat of not soiling the tube (an occurrence easily avoided by using a glassfunnel, as recommended by Dr. Christison, or which may be obviated bywiping the tube, after die introduction of the mixture, with a wisp ofpaper), while their comparative scarcity and greater cost are objectionsto their employment. (For further details concerning the reduction pro-cess, consult Dr. Christison’s Treatise , so frequently referred to.)

(b.) Characters of a pure aqueous solution of ArseniousAcid. —A clear watery solution of white arsenic may be recognized bycertain liquid reagents which give rise to peculiar precipitates, as well asby nascent hydrogen, which causes the formation of a gas (arseniurettedhydrogen) possessed of remarkable and peculiar properties. The liquidreagents which deserve notice are four only—namely, lime water, ammo-niaco-sulphate of copper, ammoniaco-nitrate of silver, and hydrosulphuricacid. Their relative delicacy, as stated by Devergie, (Med. Ley. ii. 718),and the delicacy of the nascent hydrogen test, as ascertained by Mohr(Journ. de Pharm. xxiii. 566), are as follows:—

Dilution of arsenious solution .

Lime water ceases to act at. 2*000

Ammoniaco-sulphate of copper, ditto at .... 5*200

Hydrosulphuric acid, ditto at. 200*000

Ammoniaco-nitrate of silver, ditto at. 400*000

Marsh’s nascent hydrogen test, ditto at .... 500*000

1. Lime Water. —Lime water occasions a white precipitate (arsenite oflime), with a solution of arsenious acid. The precipitate is soluble inmost acids. The impediments to the operation of this test are, a largequantity of water and free acids, which hold it in solution, and gelatinousand oleaginous liquids, which keep it suspended. The fallacies of thistest are, carbonates, oxalates, tartrates, &c. which also throw down whiteprecipitates with lime water. On the whole, it is a test of very little value.

2. Ammoniaco-sulphate of Copper. —If a dilute solution of ammoniaco-sulphate of copper be added to a solution of arsenious acid, a pale greenprecipitate (arsenite of copper, or Scheele ’s green) is obtained, and sul-phate of ammonia remains in solution. This test is prepared as follows:—Add (cautiously) liquor ammonia; to a solution of the sulphate of copper,so as to re-dissove the oxide of copper, which it at first throws down.Care must be taken not to employ too much alkali, otherwise the testwill not act. Moreover, the solution must not be concentrated, or noprecipitate will be obtained. The impediments to the action of this testare astringents, as tea, infusion of galls, &c. which prevent its actingcharacteristically. The fallacies to be guarded against, are, yellow-coloured and other organic fluids, which give a green colour, and slightprecipitate, even though no arsenic be present.

3. Ammoniaco-nitrate of silver: Hume's test. —If a solution of ammo-niaco-nitrate of silver be added to a solution of arsenious acid, a yellowprecipitate (arsenite of silver) takes place, and nitrate of ammonia remainsin solution. The precipitate is soluble in liquid nitric acid, ammonia water,and a solution of nitrate of ammonia. The mode of preparing this test isas follows:—Add a few drops of liquor ammonia: to a solution of nitrate ofsilver, so that the oxide of silver which the alkali at first throws downmay be nearly, but not entirely, redissolved. Great care is requisite toadd neither too much nor too little; for if too much be employed, thesolution will not occasion any precipitate with arsenious acid; and if too