chloride of iron, produces a dark-red solution of persulphocyanate of iron.

Solution

d. Formation of cyanide of silver. of nitrate of silver added to aqueous prussic acid throws down a white precipitate of cyanide of silver, which quickly subsides after agitation. It is not affected by cold nitric acid, but when separated from the supernatant liquid dissolves more or less completely in the strong boiling acid. The precipitate collected on a filter, washed, dried, and heated in a reduction tube, evolves cyanogen gas, which if ignited at the mouth of the tube, will burn with its peculiar rose-coloured flame.

The precipitate

ε. Decompositions of precipitate. treated with hydrochloric acid evolves prussic acid vapour, which may be received on the interior of a watch-glass moistened either with yellow sulphide of ammonium, or with a mixture of potash and sulphate of iron, as described under the head of the vapour reactions. Or a portion of the precipitate may be treated with a drop of yellow sulphide of ammonium, dried at a low temperature, and the residue, after being moistened with water, tested by a drop of perchloride of iron, when a dark red liquid will be produced, easily distinguishable from the black precipitate of sulphide of silver. Or a portion of the precipitate may be treated first with potash, then with a drop of sulphate of iron, and lastly with a little hydrochloric acid, when Prussian blue will be formed, together with white chloride of silver.

(146.) IN VAPOUROUS STATE.

The succeeding tests may be applied to the vapour of the pure liquid acid, or to the vapour produced by the action of hydrochloric acid upon precipitated cyanide of silver, or to the vapour evolved spontaneously from organic liquids or solids containing prussic acid. Organic substances which do not react satisfactorily with these vapour tests may be distilled in a water-bath, and the distillate treated similarly to the pure aqueous acid as above described.

a. Formation of Prussian blue. When a mixture of potash and sulphate of iron, smeared upon the interior of a watch

glass, or preferably on a flat glass slip (fig. 50), is exposed for a few minutes to the action of prussic acid vapour, there is produced on acidification with hydrochloric acid, a solution of the iron magma and development of Prussian blue.

B. Formation of sulphocyanate of iron. A drop of yellow sulphide

Fig. 50.

of ammonium placed on a watch-glass or glass slip and exposed for a short time to the action of prussic acid vapour, yields, when evaporated to dryness at a low temperature, a residue of sulphocyanate of ammonium, which produces a dark red colour, on the addition of perchloride of iron.

y. Formation of cyanide of silver. A drop of nitrate of silver, placed on a watch-glass or glass slip and exposed to the action of the vapour, becomes white and opaque from the formation of cyanide of silver, convertible into Prussian blue, or sulphocyanate of iron, as previously described.

When the prussic acid vapour from some organic mixture is contaminated with sulphuretted hydrogen, it produces a blackening of the silver salt; but no interference with the sulphocyanate reaction is manifested under the same circumstances.

§ XI.-STRYCHNIA.

(148.) IN PURE State.

a. Nature, solubility, &c. Strychnia is a vegetable alkaloid, having the formula C,,H,,N,0,. It is more or less freely soluble in alcohol, chloroform, benzole, and ether; scarcely at all soluble in pure water; but readily soluble in acidulated water. It is capable of uniting with and neutralising acids, to form definite crystallisable salts, of which the sulphate, nitrate, hydro

chlorate, oxalate, tartrate, and acetate are soluble in water. Most other strychnia compounds are more or less insoluble, whence solutions of strychnia salts are precipitated by a very great number of reagents, including hydrate, carbonate, iodide, sulphocyanate and chromate of potassium, carbazotic acid, phospho-molybdate of sodium, iodide of potassium with iodine, potash double iodide of mercury and potassium, perchloride of platinum, trichloride of gold, &c.

B. Appearance. Strychnia usually occurs in the form of a crystalline powder, or of well-defined prismatic crystals, either white, or of a pale buff colour. The ordinary salts of strychnia are generally met with as crystalline powders. Strychnia and its salts when heated, melt, burn with a smoky flame, and leave a carbonaceous residue.

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7. Bitter taste. The bitterness of strychnia is peculiar, and has been sometimes spoken of as metallic. Its intensity is so great, that one drop of a gallon of water, in which a grain of strychnia is dissolved, presents a recognisable bitter taste; while with part of strychnia in solution, the bitterness is well marked and persistent. The taste of strychnia salts is but slightly less intense than that of the alkaloid itself. In very dilute solutions only is the bitterness capable of partial concealment by other sapid bodies.

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d. Crystalline form. A drop or so of a spirituous or ethereal solution of strychnia allowed to evaporate spontaneously on a glass slip, furnishes a crystalline residue consisting of rectangular prisms often terminated by double or single oblique planes, and in variously modified octahedra, as shown in fig. 51. The forms deposited from a chloroformic solution are, for the most part, not well characterised.

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ε. Precipitation. Aqueous solutions of strychnia salts, containing from to part of strychnia, are precipitated by the several reagents mentioned above, either immediately or on stirring; the most delicate though least characteristic precipitants being the phospho-molybdate of sodium, and the potash

solution of hydrargyro-iodide of potassium. Hydrate or carbonate of potassium causes a gradual deposition of well-defined strychnia crystals, insoluble in ex

cess of the precipitant (fig. 51). The precipitates thrown down by iodide,sulphocyanate, and chromate of potassium, carbazotic acid, and the chlorides of platinum and gold, are also crystalline.

4. Action of acids. Strong sulphuric acid is without action on strychnia, even at and above the temperature of boiling water. Strong

Fig. 51.

nitric acid usually produces a yellow, or yellow-brown, discolouration; but is said to be without visible action on perfectly pure strychnia, although this seems doubtful.

n. Colour tests. When a little peroxide of lead is added to a fragment of strychnia, dissolved in a drop of strong sulphuric acid mixed with of its bulk of strong nitric acid; or preferably, when a little peroxide of manganese, or bichromate, or ferridcyanide, or permanganate of potassium, is added to a fragment of strychnia dissolved in a drop of strong sulphuric acid, there is produced a magnificent purple-blue colour, becoming gradually crimson, and finally reddish pink. The delicacy of this test, when special precautions are taken, is almost illimitable, less than of a grain having been stated to give the reaction. With from to of a grain it is easily obtainable. operating on small quantities, the following plan may be adopted with advantage. The dry strychnia, usually the residue of an evaporation, in which case it must be allowed to become quite cold, is moistened with the smallest sufficient quantity of strong sulphuric acid. By the side of it is next placed a minute drop of a mixture of sulphuric acid with a little very finely-powdered amorphous peroxide of manganese, and the two then brought into contact. The experiment should be made on a surface of

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white porcelain, or on a flat watch-glass or glass slip, resting on a sheet of white paper.

0. Physiological test. When a minute quantity of solid or dissolved strychnia is introduced underneath the incised skin of a small frog, well-marked tetanic convulsions are manifested by the animal, usually within a quarter of an hour; and with a strong dose almost immediately. This tetanus is said to have been produced with so small a quantity as of a grain of strychnia; but the delicacy of the test varies much with the state of the animal, freshly-caught young frogs being the most excitable.

(149.) IN ORGANIC MIXTURES.

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a. If a liquid, it is merely acidified, mixed in some cases with a little spirit of wine, filtered, evaporated nearly to dryness, and the residue extracted with strong alcohol. If a solid, it is brought into a state of fine division, and mixed with a little proof spirit acidulated with dilute sulphuric or other acid, acetic, oxalic, tartaric, &c. After digestion for some time in a water bath, the mixture is filtered, the insoluble matters washed with proof spirit, the washings added to the filtrate, the whole of the clear liquid evaporated down to a small bulk, and the residue so obtained extracted with strong alcohol. The alcoholic solution is then evaporated to dryness, the residue dissolved in a little water, the liquid filtered into a long tube or bottle, and rendered alkaline with carbonate of potassium. Two or three times its volume of ether are next added, and the whole shaken up briskly for some time. After subsidence, the ethereal solution is poured off, and allowed to evaporate spontaneously, whereby a residue is left of more or less well crystallised strychnia. This may be further purified by moistening it with strong sulphuric acid, and heating it for some time in a water bath, then diluting with water, supersaturating the acid liquid with potash, again extracting with ether, and evaporating. The final product may be examined under the microscope, by the colour and physiological tests, and by the tongue. In the above process, chloroform or benzole may be substituted for the ether.