The process here described is one which I have followed with great facility, certainty and despatch in several medico-legal cases.[[529]] It is extremely delicate; for it will detect at least a 250,000th part of arsenic in solution; and it removes from the fluid every particle of arsenic, because none can be afterwards discovered by means even of Marsh’s method. It is not subject to any fallacy. The mere formation of a brilliant coating on the copper is not evidence of arsenic being present; for as Reinsch himself ascertained, solutions of bismuth, tin, zinc, and antimony produce a coating more or less similar to an arsenical one. But the farther steps of the process entirely put aside all these sources of error. The non-formation of a metallic tarnish of copper, however, is perhaps not absolute proof of the absence of arsenic. For, according to a late statement by Drs. Fresenius and Von Babo,[[530]] “all nitrates, and various salts of mercury and other metals, render the separation of arsenic by copper difficult or even impossible.” The authors of this objection, although the paper is otherwise elaborate and detailed, have not given any particulars in illustration of so important a criticism.
Of the Tests for Oxide of Arsenic in Organic Mixtures.
The present is by far the most important of the conditions under which it may be necessary to search for arsenic in medico-legal cases; for in nine cases out of ten the subject of analysis is either some article of food or drink, the contents or tissues of the stomach, or the textures of other organs of the body into which the poison has been carried by absorption.
Accordingly much attention has been paid to this subject for some years past, and many valuable methods of analysis have been suggested, more especially since the recent discovery that arsenic, like many other poisons, undergo absorption, and is diffused by the circulation throughout the body generally. It was proved by me in 1824,[[531]] that the tests for arsenic, at that time in general use, are so fallacious when applied to complex organic mixtures as to be unfit for medico-legal investigations except merely as trial-tests; and a process was proposed, which has since undergone various modifications from others as well as myself. This process, in the form in which it was adopted in the last edition of the present work, is still applicable to a great proportion of cases; and indeed a recent modification of it has been thought by Drs. Fresenius and von Babo to be superior even yet to every other in all circumstances.[[532]] But two new methods are at present generally preferred, and probably not without reason. At least they have been much employed and with great success in numerous medico-legal researches, where the quantity of arsenic was to all appearance extremely small, and the subject of examination most complex and troublesome to bring within the sphere of analysis. And in particular they have been successfully employed to detect arsenic in those organs of the human body into which it can obtain admission only through the medium of absorption.
In the following statement I shall describe four processes only, that of Reinsch, by which the arsenic is first separated as a crust on copper,—that of Marsh, who first detaches it in the form of arseniuretted-hydrogen,—my own method, which consists in obtaining in the first instance a sulphuret of arsenic,—and that of Drs. Fresenius and von Babo, which has the same foundation.
Process of Reinsch.—This is the simplest and easiest of all. Remove in the first place any white or gray powder which can be detached from the mixture; and either subject it to the process of reduction by charcoal or soda-flux, as described at p. [203], or dissolve it in boiling distilled water and subject the solution to the three liquid reagents, p. [207], or if there be enough, examine it in both ways. If arsenic be thus obtained, it is seldom necessary to proceed any farther. But if not, cut all soft solids into small fragments, add distilled water if necessary, then add hydrochloric acid to the amount of a tenth of the whole mixture, and more if the subject of analysis be decayed and ammoniacal, so that there may be a decided excess of acid. Boil gently for an hour, or until all soft solids be either dissolved or broken down into fine flakes and grains. Filter through calico; bring the filtered fluid again to the boiling point; and then proceed as described for Reinsch’s method in simple arsenical solutions [p. [214]].
The only important precaution to be attended to in employing this process is to take care that the water, hydrochloric acid, and calico are free of accidental impregnation with arsenic. This is guarded against by applying the process to them in the first instance. I have lately employed this method of analysis with success in two medico-legal cases where the bodies had been buried for several months, and where the quantity of arsenic must have been very minute. Satisfactory evidence was obtained from a sixth part of the stomach, and also from the same proportion of the liver.
Process of Marsh.—The chief difficulties in applying the process of Marsh to complex organic mixtures arise from the tendency of oxide of arsenic to adhere with obstinacy to some organic principles in the solid state, and from the liability of the gas disengaged in the apparatus to raise organic fluids in a fine froth, which breaks up slowly, and is therefore apt to pass over into the exit-tube. Many contrivances have been devised, to meet these difficulties, especially by the French chemists and toxicologists, whose attention was turned earnestly to the subject by the investigations carried on in certain late criminal trials of great interest and importance. The various devices now alluded to were subjected to trial in 1841 by a Committee of the French Institute; who came to the opinion that the following method suggested by MM. Flandin and Danger is the most convenient and comprehensive.[[533]]
Heat the organic matter with a sixth of its weight of strong sulphuric acid; when complete solution has taken place, concentrate the fluid to a friable almost dry charcoal; add a little concentrated nitric acid gradually to this when cold, and again evaporate to dryness; then act on the residue with boiling distilled water, and a solution of a reddish-brown colour is obtained, which may be used in such an apparatus as that of Döbereiner without risk of obstruction from froth.—The arseniuretted-hydrogen, thus disengaged along with the hydrogen gas, is to be submitted to the succession of tests described in speaking of Marsh’s process for detecting arsenic in a state of simple solution [p. [212]].
This method of investigation is exceedingly precise and conclusive. The sulphuric acid aided by heat destroys organic matter sufficiently to prevent frothing in the apparatus and dissolves out arsenic from a state of combination with organic principles; and nitric acid afterwards converts any arsenic in the half-charred mass into the soluble arsenic acid. It has been employed with success in various medico-legal proceedings in France. It answers well for detecting oxide of arsenic in the viscera, muscles, and other parts of the body into which the poison has been conveyed through absorption.