The fallacies to which the test has been supposed to be liable (excluding at present that part of it which consists in the oxidation of the metal, and which renders it quite unimpeachable), are the following.— Dr. Paris says he has known an instance where a person, “by no means deficient in chemical address, mistook for it a deposit of charcoal,”[[510]] and I have known the same mistake happen in the hands of one of my pupils, a beginner in the study of medico-legal chemistry. The outer surface of a charcoal crust may be mistaken for arsenic by a careless person; but with ordinary care it is quite impossible to err if the inner surface be examined, for that of charcoal is brown, powdery, and perfectly dull.—It has been suggested to me and has been stated in print,[[511]] that the preparations of antimony yield by reduction a sublimate resembling closely an arsenical crust. But in consequence of repeated trials I am certain that no preparation of antimony, reduced either by charcoal or the black flux with the fullest red heat of the blowpipe will yield any metallic sublimate; and the same facts were observed by the late Dr. Turner.—It has even been said by Mr. Donovan that the action of the flux on glass which contains lead causes a stain similar to an arsenical crust.[[512]] If it be meant by this observation, that the lead contained in the glass usually gives that part of the tube which contains the flux a glimmering appearance and impairs its transparency, the author is correct: but it is impossible that a sublimate can be so formed.—Dr. Mitchell of Philadelphia in an elaborate paper on the process of reduction seems to consider the crust undistinguishable from that formed in similar circumstances by cinnabar.[[513]] Crusts of cinnabar, however, do not present the peculiar character possessed by the internal surface of arsenic.—Zinc, it is said, may be sublimed in its metallic state; but the sublimation of zinc requires a full white heat; which in the process for arsenic cannot be generated.—Tellurium, cadmium, and potassium sublime at a lower heat; but these metals are so exceedingly rare, that it is quite unnecessary to particularize the characters of their sublimates.—Lastly, it is said that a crust may be produced from arsenic contained in the glass of the tube. A few years ago MM. Ozanam and Idt of Lyons detected arsenic in the remains of a body which had been seven years interred; but subsequently M. Idt imagined he had discovered that the glass used in the analysis contained arsenic, and yielded it by the process of reduction. He accordingly retracted his original opinion; and the person accused of administering the poison was acquitted. An extended inquiry, however, was in consequence undertaken by the Parisian Academy of Medicine at the request of the French government. And the result was that no arsenic could be detected in the glass tubes used by MM. Ozanam and Idt; and that although arsenic is sometimes used in glass-making, and a trace of it may be retained in some opaque glasses or enamels, it cannot be detected by any process of analysis in any of the clear glass met with in commerce,[[514]] the whole arsenic being volatilized during the manufacture of the glass.
It may therefore be safely laid down that the appearances exhibited by a well-formed arsenical crust, even in the minute quantity of a 300th part of a grain, are imitated by no substance in nature which can be sublimed by the process for the reduction of arsenic.
But should farther evidence be required as to the nature of the crust, this may be obtained by subjecting it to oxidation by heat.
The best method of doing so is to heat the ball containing the flux deprived of arsenic, to attach a bit of glass tube to its end, and to draw this gently off in the spirit-flame, taking care to prevent the flux being driven forward on the crust. This being done, the whole crust, or, if it is large, a portion of it, is to be chased up and down the tube with a small spirit-lamp flame till it is all converted into a white powder. In order to show the crystalline form of the powder distinctly, let the flame be reduced to the volume of a pea by drawing in the wick, and let the part of the tube containing the oxide be held half an inch or an inch above it. By repeated trials sparkling crystals will at length be formed, which are octaedres,—the crystalline form of arsenious acid. The triangular facettes of the octaedres may be sometimes seen with the naked eye, though the original crust was only a fiftieth of a grain or even less; and they may be always seen with a lens of four powers, the tube being held between the eye and a lighted candle or a ray of sunshine, either of which is preferable to diffuse daylight for making this observation.—For the success of the oxidation test it is indispensable that the inside of the tube be not soiled with an alkaline flux: because the alkali would unite with the oxide. It is also requisite not to heat the tube suddenly to redness before the oxide is sublimed; because then the oxide is apt to unite with the glass, forming a white, opaque enamel. The physical characters of the sublimed oxide are so delicate and precise, that they may be accurately distinguished, even when those of the metallic crust are obscure, owing to its minuteness. Sometimes too, the metal may be so scanty that it is oxidated at once in the act of subliming, and never presents the appearance of a metallic crust. Although the characters of the crystalline oxide in either of these cases are very precise and distinctive, it may be right to subject it to a farther test when the metal is not previously exhibited with its characteristic properties. For this purpose it is sufficient to cut away with a file the portion of the tube which contains the sublimate, to boil it in another tube with a few drops of distilled water till the sublimate disappear, and then to test the solution with one of the fluid tests to be presently described, the ammoniacal nitrate of silver.
After all that has been recently written as to the old and newer processes for detecting arsenic, I must nevertheless avow my conviction, that for solid arsenic no test is, for medico-legal purposes, at once so satisfactory, convenient, and delicate as the test of reduction, especially with the addition of the supplementary test of oxidation. That other methods are still more delicate may be readily granted. But where the suspected substance is in the solid form, what possible occasion can there be for a method more delicate than one which will detect a 300th part of a grain? A method ten times less so would meet every case in actual practice.—A variety of supplementary tests have been proposed. But they are all greatly inferior in facility, or conclusiveness, or both, to the process of oxidation, and ought therefore to be expelled from medico-legal practice,—not even excepting the alliaceous odour of metallic arsenic in the act of subliming, a character, the fallaciousness of which was long ago pointed out by myself as well as others, and to which a preposterous importance has been attached in some late inquiries. The reader will find in the last edition of this work an attempt to estimate the value of various tests supplementary to that of reduction. This disquisition is now omitted, as it seems no longer necessary.
Of the Tests for Oxide of Arsenic in Solution.
Oxide of arsenic in a state of solution may be detected in one of four ways; by what are called the liquid tests; by precipitating it with one of these, and subliming metallic arsenic from the precipitate, which method is usually termed the reduction process; by Marsh’s method, which consists in disengaging it in the form of arseniuretted-hydrogen gas, and decomposing the gas by combustion; or by the method of Reinsch, in which metallic arsenic is deposited on the surface of copper, and then separated by heat for farther examination.
Process by Liquid Reagents.—The first method is by the employment of several liquid tests, which cause in the solution peculiar precipitates. Many such tests have been proposed; but the most characteristic and precise are hydrosulphuric acid, ammoniacal nitrate of silver, and ammoniacal sulphate of copper. The indications of each of the three tests must concur, otherwise, in a medico-legal case, no one can be entitled to speak with certainty to the existence of arsenic. But when they do concur, the evidence is unimpeachable. When this method of analysis is followed, corresponding experiments ought always to be made with the water that is used for diluting or otherwise preparing the subject of examination, or with distilled water, if the article be already sufficiently aqueous. This precaution is necessary on account of the risk of accidental impregnation of the water or other reagents with arsenic.[[515]]
Hydrosulphuric acid [sulphuretted-hydrogen] is obtained by decomposing proto-sulphuret of iron with diluted sulphuric acid in such an apparatus as is represented at Fig. 5. And the gas may be either applied directly to the suspected fluid, or condensed in distilled water, and thus kept in store for occasional use in the liquid shape. Before applying this test, the suspected fluid must be acidulated with acetic or hydrochloric acid; because an excess of alkali prevents the action. And if an acid be indicated by litmus in the fluid, neutralization, or slight supersaturation, with potash must be effected, before adding acetic or hydrochloric acid; for if the acidity should happen to be owing to an excess of sulphuric or nitric acid, the test is decomposed, and yellowish-white sulphur deposited.—These precautions being taken, hydrosulphuric acid occasions a sulphur-yellow or lemon-yellow precipitate. If the arsenical solution, however, be very weak, a yellow colour merely is struck, because the precipitate, which is sesqui-sulphuret of arsenic, is dissolved by the excess of the test; but it separates after ebullition, or a few hours’ exposure to the air. Co-existing animal and vegetable principles sometimes enable the fluid to retain a minute portion even after ebullition, so as to acquire a yellow milkiness; but they do not in any case prevent the test from producing the yellow colour. Acidulation with acetic or hydrochloric acid favours its subsidence in all cases; and according to Mr. Boutigny, alkaline sulphates, muriates and nitrates have the same effect.[[516]] Hydrosulphuric acid is so delicate as to act on the oxide in a hundred thousand parts of water. The proper colour of the precipitate is lemon or sulphur-yellow; which, when vegetable or animal matter is present, acquires a shade of white or brown.
It is not liable to any material fallacy. The salts of cadmium yield with it precipitates nearly of the same colour: but they are exceedingly rare; and the precipitate, unlike sulphuret of arsenic, is insoluble in ammonia.—The salts formed by selenic acid, if decomposed by another acid, also yield yellow precipitates; but these salts are extremely rare.—The salts of peroxide of tin give a dirty grayish-yellow precipitate; which however ammonia turns brown.—A lead solution acidulated with hydrochloric acid gives at first a yellow precipitate; but this becomes brownish-black when more gas is transmitted.[[517]] The contents of the human intestines sometimes yield a yellowish precipitate though no arsenic be present; and it is dissolved, like sulphuret of arsenic, by ammonia.[[518]] The tartrate of antimony and potash (tartar-emetic) does not form, as was once thought, any source of fallacy, the antimonial precipitate having always a tint of orange-red; besides it is not, like sulphuret of arsenic, soluble in carbonate of ammonia.—Other fallacies exist, unless the test be used with the precautions mentioned above. But these need not enumeration here.