it has become colorless. If necessary, the evaporation must be carried to dryness, and the residue dissolved in a little water. The colorless liquid is then feebly acidulated with hydrochloric acid, and a drop of sesqui-chloride of iron added; the liquid assumes a blood red color if the distillate contained hydrocyanic acid (Liebig). This test is founded on the observation that hydrocyanic acid forms with sulphide of ammonium, sulphocyanide of ammonium, which produces, with the sesqui-salts of iron, the peculiar blood red color. This reaction is also very delicate, and possesses the advantage of admitting of a concentration of the liquid under examination by heat.

[In making this test, particular care should be taken to remove the excess of sulphide of ammonium, as indicated, by evaporation, and not to add the salt of iron until the liquid has become quite colorless; for, if this precaution be not observed, a black precipitate of sulphide of iron is thrown down, which interferes with the reaction. Small quantities of free acid do not change the blood-red color of the liquid, but additions of larger quantities make it paler. Nitric acid destroys the coloration; ammonia decolorizes the liquid immediately, and throws down the hydrate of the sesqui-oxide of iron. Reducing substances, ex. gr. sulphureted hydrogen, protochloride of tin, metallic iron, cause the disappearance of the color, by converting the sesqui-oxide of iron into the protoxide.

Before subjecting the distillate to this test, it is absolutely necessary to convince one's self of the absence of acetic, formic, and meconic acids, which also possess the property of imparting a red color to

the sesqui-salts of iron. It is, therefore, advisable to test the distillate, previous to its treatment with sulphide of ammonium, with a drop of sesqui-chloride of iron; if a red color is produced, this reaction has either to be abandoned, or the above-named acids. have to be removed-a problem of considerable difficulty.

The delicacy of the test is very great. It gives a distinct reaction, when the mixture of a proto and sesqui-salt of iron produces only a faint blue coloration, which is by no means so characteristic as the blood-red color produced by this test. To detect, after this method, the presence of a very small quantity of hydrocyanic acid, the substance is placed in a watch-crystal, and another watch-crystal, at the bottom of which a drop of sulphide of ammonium has been placed, inverted over it. A very gentle heat is applied to the lower glass, for about ten or fifteen minutes. The upper glass is then removed, the liquid which it contains evaporated to dryness at a very gentle heat, and the residue moistened with some sesquioxide of iron. It must be borne in mind that the reaction is produced only by free hydrocyanic acid; compounds ex. gr. cyanide of silver, have previously to be moistened with a drop of hydrochloric acid.]

$77. Lassaigne recommends to add to the distillate a few drops of caustic potassa, and, immediately afterwards, a few drops of a solution of sulphate of copper; the mixture is then feebly acidulated with hydrochloric acid. If the distillate contained hydrocyanic acid, a greenish-yellow (protocyanide of copper), or white (subcyanide of copper) precipitate

remains undissolved; but, if no hydrocyanic acid was present, the precipitate, consisting of pure hydrate of oxide of copper, is wholly re-dissolved. According to Berzelius, a similar reaction is produced by hydriodic acid.

[A person not experienced in chemical manipulations is liable to obtain wrong indications with this reagent. Too little potassa, or too much sulphate of copper, prevent the formation of the precipitate; the addition of too much hydrochloric acid, on the other hand, causes the re-solution of the whole precipitate; even if hydrocyanic acid was present.-Schneider.]

[$ 78. Another portion of the distillate is tested with a few drops of nitrate of silver. This reagent produces in acid, or neutral liquids containing hydrocyanic acid, a white, curdy precipitate, which is soluble in ammonia; from the ammoniacal solution the cyanide of silver is re-precipitated by an excess of nitric acid. This white precipitate is not characteristic of hydrocyanic acid. The same reaction is produced by hydrochloric, hydriodic, hydrobromic, carbonic, phosphoric acids, and several other substances. It is, therefore, necessary to prove, by other experiments, that the precipitate really contains cyanide of silver. For this purpose, the precipitate is thoroughly dried, and the dry mass introduced into a small test-tube, closed at one end, and narrowly drawn out at the other; on applying heat, which may be raised to redness, cyanogen gas escapes, which, when kindled, burns with a purple. flame. Orfila states that one grain of cyanide of silver, treated in this manner, evolves sufficient gas to show the red flame very distinctly.

To get rid of hydrochloric acid and other substances which interfere with this reaction, Otto recommends to rectify a portion of the original distillate by re-distillation over powdered borax or carbonate of lime. Hydrochloric acid, etc., are retained by the borax, or the carbonate of lime, while hydrocyanic acid passes over, and may be detected in the distillate by precipitation with nitrate of silver.]

§ 79. It is sometimes very convenient to fix a small quantity of hydrocyanic acid, which may occur in a distillate, so as to prevent its volatilization, and form a compound which may afterwards be easily tested with reagents. This is effected by agitating the distillate with some red oxide of mercury; a solution of cyanide of mercury is obtained which may be evaporated without suffering decomposition. If hydrochloric acid was present, protochloride of mercury (corrosive sublimate), is formed at the same time, and, in this case, ammonia causes a white precipitate, owing to the decomposition of the chloride while the cyanide remains unchanged, and nitrate of silver throws down chloride of silver, leaving the cyanide unaffected.

[§ 80. Instead of simply condensing the vapors in a receiver as in the process above (§ 74) given, Orfila connects the retort with a delivery-tube, bent at a right angle, the vertical leg of which is about three and a half feet long, and dips into another tube of about the same length, which is filled to about twothirds with a solution of nitrate of silver. The retort is heated in a sand-bath, and the liquid kept in ebul lition for about an hour. The precipitate which is

formed in the silver solution is allowed to settle to the bottom, the supernatant liquid is poured off, and some pure, cold nitric acid added to the precipitate; this dissolves the carbonate of silver. The cyanide is then collected on a filter, dried at 212° (weighed, if it is thought desirable), and tested by heating in a glass-tube (v. § 78). A portion of the dried precipitate may be tested in the following manner: it is mixed with about half its weight of chloride of sodium, a little water added, and the whole boiled for a few minutes; chloride of silver and cyanide of sodium are formed, which latter may be tested with green vitriol (§ 75). An excess of chloride of sodium has to be avoided, lest the cyanide of silver enter undecomposed into solution, and make the reaction with green vitriol indistinct.]

[8 81. Whether these operations have given a positive or a negative result, it is always advisable to subject the contents of the retort to a further treatment. The mass, if too pasty to admit of filtration, is stirred with some distilled water, filtered, and the filtrate precipitated with nitrate of silver. The precipitate, after having been well washed with distilled water, is treated with dilute nitric acid in the cold; the carbonate, phosphate, etc., are dissolved, and the residue consists essentally of chloride and cyanide of silver. It is tested for the presence of the latter by the processes given above (§ 78).]

§ 82. If by the method above described (§ 74), we obtain a distillate in which, by the use of proper reagents, the presence of hydrocyanic acid can be demonstrated, this can by no means be considered as a proof for the presence of hydrocyanic acid or,