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of an infusion of tobacco-leaves, and, in my laboratory, it has repeatedly been extracted from a cigar.
$ 129. If the presence of opium has to be established, the operations may be so conducted that the resulting solution contains a salt of meconic acid; the latter is then easily detected by its reaction with sesquichloride of iron. The substances are treated with dilute alcohol and a few drops of hydrochloric acid; the extract is concentrated by evaporation; the residue dissolved in water; the aqueous liquid separated by filtration from insoluble matters; the filtrate mixed with magnesia usta in excess, and heated to ebullition; then filtered. The resulting liquid contains meconate of magnesia. It is acidulated with hydrochloric acid, and mixed with a few drops of sesquichloride of iron. In presence of meconic acid, the liquid assumes a dark, brownish-red color. Since opium is the only substance in which meconic acid occurs, to have proved the presence of this acid is to have proved the presence of opium.
[Though the detection of meconic acid is undoubtedly a conclusive proof for the presence of opium, still Stas' method, by which the morphine is obtained, deserves, in my opinion, the preference, and for this simple reason, that some kinds of opium, according to Robiquet, do not contain the slightest trace of meconic acid. It might, therefore, happen that, notwithstanding large quantities of opium have been administered, the analyst might fail to detect its presence.
In regard to the blood-red color which meconic acid produces in solutions of the sesquisalts of iron, it must be remembered that sulphocyanides and acetates give a very similar reaction. In the case of an acetate, the color is destroyed on addition of dilute acids, and, on boiling, a brownish-red precipitate is thrown down. In the case of meconic acid, or a sulphocyanide, the color does not disappear on addition of dilute acids, not even on boiling. Terchloride of gold destroys the color produced by a sulphocyanide, but not that produced by meconic acid.--(Vogel.)
To obtain additional proof as to the nature of the acid, the filtrate containing the meconate of magnesia may be precipitated with nitrate of silver. White meconate of silver is thrown down, which is collected on a filter, and washed with water. During this operation, it is frequently converted into white lustrous crystalline scales. It dissolves completely in moderately strong nitric acid; on applying heat to the solution, violent effervescence takes place, cyanide of silver is precipitated, and the liquid contains oxalate of silver.]
$ 130. It has been repeatedly remarked in the preceding pages that, before proceeding with a chemico-legal investigation, circumstantial evidence has usually thrown some light on the matter, and pointed out the nature of the poison. It may, however, happen that no such indications have been previously gathered, and the question arises: “How shall we operate under these circumstances ?” From what has been said in regard to the detection of the individual poisons, or groups of poisons, a general method may be deduced, which leads to the detection of any poison, and which does not even require a division of the material under examination.
The substances under examination, if not possessing an acid reaction, are slightly acidulated, and placed in a retort provided with a receiver. A few pieces of sulphur are added, and the retort heated over a water-bath until a few grammes have passed over. The distillate is tested for hydrocyanic acid (875). If no hydrocyanic acid is found, the heat may be slightly increased in order to cause the phosphorus, which might be present, to combine with the sulphur, and to produce the characteristic light. Both the distillate and the pieces of sulphur are afterwards examined for phosphorus (v. $ 96). If no phosphorus is found, the contents of the retort are examined for the presence of alkaloids, after the method of Stas (v. $ 116, etc.) As in this process no poisonous substances of any kind are introduced into the mass, it may finally be examined for mineral poisons, if no alkaloid should have been detected. For this purpose, all the different residues, extracts, and liquids, are united; ether and alcohol are removed by evaporation, and the remaining mass treated as described $ 59, etc.
ON THE EXAMINATION OF BLOOD-STAINS.
$ 131. It is sometimes of great importance to identify spots of blood on clothes, on the soil, furniture, or on the instrument with which death has been inflicted. Though the identification is very easy in some cases, in others again, it is attended with considerable difficulties, and a satisfactory result only to be arrived at with the greatest care and circumspection. This will be readily conceived if we recall to our minds the great variety of circumstances under which an examination of this kind may be asked for. The stains may be on white or colored clothes; they may be on cotton or linen fabrics, on fibres free from nitrogen, or they may occur on silk or wool, on fibres containing nitrogen; they may be recent, or they may be old; they may be unaltered, or they may have been partially removed by washing with water; the soiled instrument may be of iron or steel, or any other metal, it may have been bright or rusty, etc., and all these different circumstances will, of necessity, influence to some degree the modus operandi.
We shall confine our remarks to the chemical ex