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stomach, are most conveniently treated with a mixture of cyanide of potassium and carbonate of soda, as described in $ 25.]
$ 8. If arsenious acid or fly-powder cannot be found in the solid state, the mass (the contents of the stomach and intestines, and even these organs themselves, the food, the vomited matters, etc.) must be subjected to a further treatment; but, even if particles of arsenious acid or fly-powder have been found, it is, in most cases, advisable, or even necessary, to extend the investigation for the presence of arsenic in these substances still farther, if it were only to form an opinion as to the amount of the poison. With reference to the course of action to be pursued, we may remark here, that the food, the contents, the vomited matters, etc., have, each of them, to be treated separately. The separation of the contents of the intestines from those of the stomach may be of special interest in deciding the question, whether arsenic has passed from the stomach into the intestines.
It is furthermore advisable to set a portion of the mass aside, and to reserve it, so that in case of an accident the analyst may be enabled to begin the examination anew. It may be said, without hesitation, that where there is arsenic undoubtedly present, half of the mass will be sufficient to detect it with certainty, at least in 99 cases out of 100. Before dividing the mass for this purpose it should be uniformly mixed.
$ 9. The first operation consists in destroying the whole of the organic matter and dissolving the whole of the arsenic.
The mass under examination is (after having been
cut into shreds, if necessary), introduced into a porcelain dish, and about as much pure concentrated hydrochloric acid added as is equal to the weight of the dry substance contained in the mass, or a little more, and enough water to form the whole into a thin paste. If the substance has previously been mixed and stirred with water, as mentioned above, for the purpose of detecting particles of solid arsenious acid, it may be necessary to remove part of the water by evaporation in the water-bath, before adding the hydrochloric acid; this however, will be but rarely required. The dish is then placed into a water-bath, or steam-bath, and to the hot mass some pure chlorate of potassa added, at intervals of five minutes, in portions of about half a drachm each (2 grammes), until the whole is converted into a perfectly homogeneous, light yellow, liquid mass. When this point is reached, another addition of about 2 drachms (8 grammes) of chlorate of potassa is made, and the heat continued until the smell of chlorine has entirely disappeared. If the liquid should become too concentrated by evaporation, a little water is from time to time added. The mass is now allowed to cool; the contents of the dish are thrown on a moistened filter of white paper, free from smalt; when the quantities are large, a white linen cloth may be substituted, and the residue washed with hot water. The washings are collected in a separate vessel; if they yield too much liquid, they are evaporated on a water-bath in a porcelain dish, and, after having been sufficiently concentrated, united with the first filtrate. The whole of the liquid should amount to about a pint.
The chemist will readily understand the object of this operation. The chlorine, or the compounds of chlorine, which are formed and set free by the action of the hydrochloric acid on the chlorate of potassa, act decomposing on the organic substances, and convert the arsenic, in whatever form it may occur, even when present as sulphuret, in arsenic acid.
In consideration of the fact that, from a liquid containing arsenious or arsenic acid, and hydrochloric acid, chloride of arsenic may escape in the gaseous state, the analyst might feel inclined to perform the operation, viz.: the treatment of the mass with hydrochloric acid and chlorate of potassa, in a tubulated retort, connected with a receiver. In regard to this, experience has shown that, at the temperature to which the mass is raised, when heated in a water, or vapor-bath, no perceptible quantity of arsenic is lost, but that an evaporation of the concentrated liquid is not admissible. There is, however, no objection to using a retort. The retort is placed on a wire-gauze over the furnace, and a moderate heat applied. The distillate must be examined sepa- . rately. If it is deemed necessary to concentrate the liquid by evaporation, it has to be done in a retort, the temperature being kept below the boiling point.
$ 10. The next operation is the precipitation of the arsenic from the liquid as sulphide. The liquid is poured into a spacious beaker-glass, or a flask, and a current of washed sulphureted hydrogen gas slowly passed through it; when the liquid is perfectly saturated with gas, it is heated to about 140°-170° F., which is done by placing the vessel containing it into hot. water; it is then allowed to cool, the gas all the while
passing through it, and finally set aside, well covered, at a moderately warm place, for about twenty-four hours. If, after this time, the odor of sulphureted hydrogen should be but faint, sulphureted hydrogen gas is again passed into it, and the liquid again left standing for some time, at a moderately warm place. By this means, and only by this means, viz.: by a continued and repeated treatment with sulphureted hydrogen, the whole of the arsenic is removed and precipitated as sulphide.
With regard to this operation, it must be borne in mind that arsenic acid, and in this state the arsenic is contained in the liquid, is but slowly decomposed by sulphureted hydrogen. Heat aids and facilitates the decomposition of the acid and the precipitation of the sulphide.
The precipitate contains, besides sulphide of arsenic, organic matter and, if lead, copper, mercury, and antimony were present in the liquid, the sulphides of these metals. The color of the precipitate is, in this respect, suggestive.
If there is only a very minute quantity of arsenic present, it may happen that, after the treatment with sulphureted hydrogen, in the above indicated manner, not the slightest trace of a precipitate is deposited. It then forms on evaporation, in the measure as the acid becomes more concentrated; and hence, in a case like this, it should never be omitted to evaporate the liquid, and to treat the concentrated liquid repeatedly with sulphureted hydrogen gas.
On the other hand, the formation of a precipitate, after the passage of a current of sulphureted hydrogen gas, does not warrant the inference that arsenic is present, since, almost invariably, a deposition of organic matter, even in absence of arsenic, takes place. If sulphureted hydrogen is passed into the liquid, which is filtered off from the first precipitate, caused by sulphureted hydrogen, another precipitate will be formed, even if it no longer contains any arsenic.
Since arsenious acid is much more readily, and within a much shorter time, decomposed by sulphureted hydrogen than arsenic acid, it has been proposed to reduce the arsenic acid, contained in the liquid which results from the treatment of the original substance with hydrochloric acid and chlorate of potassa, by means of sulphurous acid to arsenious acid before passing sulphureted hydrogen into it. For this purpose, the liquid is heated in a waterbath, and so much of a concentrated solution of sulphurous acid, or sulphite of soda added that the odor of sulphurous acid is distinctly observable; the heat is then continued until the odor has entirely disappeared. Woehler himself, with whom the proposition originated, afterwards abandoned the use of sulphurous acid.
$ 11. The next operation consists in collecting and purifying the precipitate. After the liquid, still highly charged with sulphureted hydrogen, has cooled, the precipitate is transferred to a small filter, and well washed, at first with water containing some sulphureted hydrogen.* To remove other me
* Formerly it was laid down as a rule to set the liquid aside before filtering it, until the whole of the sulphureted hydrogen had evaporated, and the principal object of placing the vessel at a moderetely warm spot, was to facilitate the escape of the gas. But it has