investigation. We shall pursue here the same way as in the case of arsenic, and assume the case to be of the most complicated kind, where the metallic poison is mixed with organic matters, as it will be when vomited matter, contents, etc., are to be examined. The method for cases of aless complex nature is then easily derived from this.

In embracing the metals to be treated of in this chapter under one head it was not meant to intimate their simultaneous occurrence, but it was done simply to avoid frequent repetitions, and because such an arrangement offers great advantages for cases where there is no indication whatever as to the nature of the poison. The remark has been made on several occasions in the preceding chapter, that such and such a method for the detection of arsenic could not be recommended, because it did not lead at the same time to the detection of other poisonous metals. To the method indicated as the best, this deficiency does not apply; it leads to the detection of all the above-named metals, if the analyst chooses to take account of them, that is to say, if he looks for them at the proper stage in the course of the investigation. Up to a certain point, the method for the detection of the metals in question coincides with the one given for the detection of arsenic, and, unless we are convinced of the absence of the latter, due attention should be paid to its possible occurrence.

59. To make random experiments for the presence of one or the other of the metals will, as a general thing, be a useless task, because the soluble metallic compounds are almost invariably converted into insoluble ones when organic matters are pre

sent. Copper may possibly betray its presence by a greenish color, and by the blue coloration produced by ammonia. It is, therefore, best to subject the substances under examination directly to the treatment with hydrochloric acid and chlorate of potassa, in exactly the same manner as described for arsenic in § 9. The free chlorine is then removed by a continued gentle heat, the residue diluted with water and allowed to cool; the mass is then filtered, the residue well washed with water, and put aside for further examination, if it should appear desirable.

The filtrate, provided the operation to have been carefully conducted, is a perfectly limpid liquid, of a yellowish color, in which, without further preparation, the presence of some metals, if not occurring in too small a quantity, may be satisfactorily detected by the proper reagents.

A portion of the liquid is mixed with ammonia in excess; if it becomes blue, the presence of copper is indicated. The color is most clearly seen on holding the test-tube against a sheet of white paper. The blue color assumes a greenish tinge in proportion as the yellow color of the liquid was deeper.

Ferrocyanide of potassium, added to another portion of the filtrate, produces a red precipitate, or merely a red color of the liquid, even in presence of a very small quantity of copper; the presence of iron makes the reaction indistinct.

The presence of lead may be demonstrated by sulphuric acid. To a portion of the filtrate some rectified, concentrated (or better, slightly diluted) sulphuric acid is added, drop by drop; a white precipitate, or turbidness, indicates lead.

In a similar manner, the presence of mercury may be shown by protochloride of tin, which causes a white turbidness, and by metallic copper, which becomes amalgamated (see § 64).

§ 60. Whether, by these reagents, indications are obtained, or not, the filtrate is now treated with sulphureted hydrogen, in the manner described § 10. The metals under consideration, with the exception of zinc, are precipitated as sulphides, provided the liquid was sufficiently dilute. The liquid, saturated with the gas, is allowed to rest at least twenty-four hours, to effect a complete precipitation and separation of the metallic sulphides.

We have next to take the color of the precipitate into consideration. If the precipitate consists of sulphide of lead, sulphide of copper, or sulphide of mercury, the color is dark; it is light, if the precipitate consists of sulphide of antimony, or sulphide of tin, or if none of the metals in question are present (compare § 10). The sulphides of lead and mercury are of a pure black color; they are pulverulent, and settle readily to the bottom. The sulphide of copper is blackish-brown, hydrated, and settles down less easily; the liquid retains, for a considerable time, a brown coloration. The sulphide of antimony is not precipitated with its characteristic orange-red, but with an impure yellow color. In presence of lead, the precipitate is, at first, usually of a cinnabar-red color (chlorosulphide), which becomes gradually black (pure sulphide). If, from this and previous reactions, the presence of lead has been inferred, the residue. from the treatment with hydrochloric acid and chlorate of potassa is once more washed with hot water,

to dissolve any chloride of lead which it might contain. This residue may subsequently be examined for the presence of sulphate of lead.

$61. The precipitate must now be collected. Let us suppose it to be of a dark color. It will be found very convenient to avoid, altogether, the use of filtering-paper, and, with the necessary precaution, it may be avoided. The sulphides of lead and mercury are deposited in a pulverulent state, and it is a very easy matter to pour off the supernatant liquid, and to wash the precipitate by repeated additions of water; glasses of conical shape are advantageously employed in this operation. The precipitate is finally transferred from the glass into a porcelain dish, and dried at a moderately warm spot. The sulphide is thus obtained in a very pure condition. The washings are all united in a large beaker-glass, not to lose traces of the precipitate which might have been poured off with the liquids. The sulphide of copper is not deposited in an equally compact state; on pouring off the supernatant liquid, small, flaky particles of the precipitate are very apt to pass off with it. But, with patience, which conquers many diffi-. culties, also this one may be overcome, and the washing of the precipitate be effected by decantation.

The precipitate may, also, very conveniently be collected in a funnel, into the barrel of which a small plug of fibrous asbestos is loosely fitted. A few experiments will very soon show the necessity of having the plug but loosely fitting, and not pressed into the barrel, to admit of ready filtration. A few drops of water poured on the plug, and which serve at the same time to moisten the asbestos, show whether it

is of the proper condition. The liquid is removed from the precipitate, as much as possible, by decantation, and the residue stirred up and poured into the funnel. Filtration proceeds but slowly, but the solid matter collects very well in the lower portion of the funnel, and does not, to any extent, adhere to the glass. It is then very easily washed. The moist precipitate is removed from the funnel by means of a small spatula of platinum, and placed into a porcelain dish, ready for dessication.

To the precipitate contained in the dish, some moderately concentrate nitric acid is added, and a gentle heat applied.

§ 62. Copper.-If the precipitate consisted of sulphide of copper, nitric acid effects complete solution, and the resulting liquid is of a blue color. The liquid is mixed with a little sulphuric acid (to remove the nitric acid), and evaporated till nearly dry; a bluish mass is obtained. If, on evaporation, the mass does assume a dark color, owing to the presence of organic matters, a little chlorate of potassa is added. A similar addition, together with a corresponding amount of sulphuric acid, has to be made, if the precipitate has been collected on a filter, and then been treated with nitric acid. The residue here, as in the other case, is of a pure, greenish-blue color. The residue is then dissolved in water. In the resulting solution, the presence of the copper is demonstrated by the usual reagents. Ammonia, added in excess, imparts to the liquid a deep blue color. Ferrocyanide of potassium causes a brownish-red coloration, or brownish-red precipitate. A piece of