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liquid filtered off from the precipitate which was caused by sulphureted hydrogen. If the substances under examination contained zinc, it will be found in this liquid. It is made slightly alkaline by addition of ammonia, and then mixed with sulph-hydrate of ammonia. In presence of zinc a precipitate of a dirty greenish-grey color is thrown down. The precipitate is allowed to settle, and then dilute acetic acid added until the liquid shows an acid reaction; the whole is well stirred up and left to rest for some time. The precipitate becomes gradually of a lighter color, owing to the entering into solution of the sulphide of iron, which, together with the sulphide of zinc, was precipitated by the sulph-hydrate of ammonia. Phosphates of the alkaline earths, if present, are also dissolved. The precipitate is collected, washed with water, dried, and calcined (with the filter) in a porcelain crucible. The calcined mass is dissolved in sulphuric acid, with addition of some hydrochloric or nitric acid, the solution freed from the excess of acid by evaporation, and diluted with water. The dilute liquid, which is a watery solution of sulphate of zinc, is filtered, and the filtrate tested for zinc with the proper reagents. A portion of the liquid is mixed with concentrated sulphureted hydrogen water; no precipitate is produced; but on adding a solution of acetate of soda, sulphide of zinc is thrown down in the shape of a white flocculent precipitate. Sometimes a precipitate is produced by sulphureted hydrogen alone; but this is only the case when the excess of sulphuric acid was entirely removed by evaporation. Caustic soda precipitates, on careful addition, white hydrate of zinc, readily soluble in an excess of the reagent. In this solution sulphureted hydrogen causes a white precipitate of sulphide of zinc. Carbonate of soda precipitates on boiling carbonate of zinc. The whole of the solution may be treated in this manner, the precipitate be collected, ignited, and weighed. The carbonate, by ignition, is converted into oxide.

$ 71. If the examination has led to the detection of a fixed metal, the residue from the treatment of the substances under examination with chlorate of potassa and hydrochloric acid may be dried, carbonized, and incinerated, and the metal be quantitatively determined. In this residue chloride of silver will be found in case a salt of silver was administered. By treating the residue with ammonia the chloride of silver is dissolved, and may be precipitated from the solution by an addition of nitric acid. The whole amount of silver cannot thus be ascertained, since a certain portion of the chloride will have entered into the solution which is filtered off from the original residue, owing to the presence of the large quantity of chloride of potassium.

CHAPTER III.

ON THE DETECTION OF HYDROCYANIC ACID.

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$ 72. HYDROCYANIC acid belongs to the class of parcotic poisons. Though the pure acid is not easily accessible to the public at large, still cases of accidental and willful poisoning with the medicinal acid, with cyanogen compounds, and with the various essential oils and distilled waters, into the constitution of which prussic acid enters as a constituent, are of frequent occurrence.

Whether the pure acid, on one of its components, has been administered, the object of the analyst's investigation is the detection of hydrocyanic acid, this substance being the toxic principle; what the other components of the compound are, it is, in most cases, immaterial to know. The method to be pursued is the same for the pure acid and its compounds.

$ 73. If the quantity of hydrocyanic acid is not too small, and no other strongly odorous substances are present, its presence is usually betrayed by its characteristic odor of bitter almonds; particular attention should, therefore, be paid to this point, at the post-mortem examination.

[This, however, is by no means a constant phenomenon.—(Pelikan).

According to the experiments of Pelikan,* this acid may always be detected in the contents of the stomach, by the processes detailed below (S 74), whenever the peculiar odor is distinctly perceptible. But, if the odor was wanting, or but indistinctly perceived, he never obtained indications of its pre- , sence by chemical reagents. The peculiar odor must, therefore, be considered as one of the most important indications of the presence of this poison, though not the safest, or most conclusive.

Here, as in‘all cases of medico-legal analysis, it is not on one observation alone that a judgment should be formed, but the opinion must be based on the agreement of all the various tests.

To detect the presence of hydrocyanic acid in substances of various kinds, ex. gr. in food, contents of the stomach, we proceed as follows:]

$ 74. The substances, if solid, are cut into small pieces or shreds, and made into a thin paste with distilled water. If the mass does not show acid reaction, it is slightly acidulated with sulphuric acid, introduced into a retort, the latter placed over a water-bath, and distilled until several grammes of liquid have passed into the receiver.

In the distillate the presence of hydrocyanic acid may be shown very conclusively by the application of reagents, and, if the

quantity is not too small, be recognized by the smell.

* Zur Toxikologie der Cyan Metalle, von Prof. Eugen Pelikan, in Vierteljahrschrift für die pract. Heilkunde. Prag. 1856.

$ 75. A part of the distillate is introduced into a small test-tube, and a few drops of caustic potassa, and afterwards a few drops of a solution of green vitriol added; the mixture is stirred with a glass-rod, until the precipitate assumes a greenish color. Ferrocyanide of potassium is formed. The liquid must have an alkaline reaction. Hydrochloric acid is now added, drop by drop, to dissolve the excess of the black oxide of iron; an addition of some sesqui-chloride of iron by means of a glass rod, will sometimes make the reaction more distinct, but is not indispensable, since there is usually sufficient of the sesquioxide formed by the action of the air. If the distillate contained any hydrocyanic acid, there remains a blue residue of Prussian blue undissolved; but if the quantity is very small, the liquid assumes only a blue, or greenish blue color, and, after some time, a blue precipitate is deposited. This reaction is so delicate and characteristic, that it makes every other test almost superfluous.*

[It is very important that the liquid have an acid reaction, because many organic substances produce, with the salts of iron, a bluish-colored liquid, or even a bluish precipitate, but these disappear on addition of an acid.]

$ 76. To another portion of the distillate, a few drops of sulphide of ammonium are added, and the mixture heated, very gently, in a small capsule, until

* I cannot recommend the method of heating the liquid containing the hydrocyanic acid with caustic potassa and green vitriol, for the purpose of facilitating the formation of ferrocyanide of potassium, because the precipitate of black oxide becomes very compact, and does not easily dissolve in hydrochloric acid.

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