The oxide of arsenic has a specific gravity of 3·729, according to the experiments of Dr. Ure,—of 3·529 when opaque, according to Mr. Alfred Taylor, and 3·798, when translucent. Very incorrect notions prevail as to its taste. It was long universally believed to be acrid,[493] and is described to be so in many systematic works and express treatises; but in reality it has little or no taste at all. The reader will find some details on this point in a paper I published in the Edinburgh Medical and Surgical Journal.[494] In the present work it is sufficient to observe, that I have repeatedly made the trial, and seen it made at my request by several scientific friends, and that, after continuing the experiment as long, and extending the poison along the tongue as far back, as we thought safe, all agreed that it had scarcely any taste at all,—perhaps towards the close a very faint sweetish taste. It appears to me that the experiments made on that occasion might have set at rest the question as to the taste of arsenic, and corrected an important error long committed by systematic authors in chemistry as well as medical jurisprudence. And accordingly in this country the truth is generally known.[495] Professor Orfila, however, continues to repeat the error; for even in the last edition of his Toxicologie he says it has “a rough, not corrosive, slightly styptic taste, perceptible not for a few seconds, but persistent, and attended with salivation.”[496] These sensations must be either imaginary or the indications of an organ peculiarly constituted. It is impossible to make satisfactory experiments with safety on its impressions on the back of the palate. But we may rest assured that in general it makes no impression there at all; for it has been often swallowed unknowingly with articles of food. Not a few have in such circumstances noticed merely its grittiness, and thought there was sand in their food. Two instances only am I hitherto acquainted with, where an acrid sensation would seem really to have been experienced in the act of eating or swallowing. In one of these, noticed in Rust’s Journal, the individual who was poisoned, could not finish the poisoned dish on account of its unpleasant, very peppery taste.[497] In the other case, which was lately communicated to me by Mr. Hewson of Lincoln, the individual, who was poisoned by arsenic dissolved in his tea-kettle,—happening in the first instance to wash his mouth with the water,—observed at the time to his daughter, that it had a very odd taste; which subsequently was called a burning taste. These facts, however, are evidently not altogether satisfactory. It is not improbable that, in an ex post facto description, the reporters, as others in the same circumstances have clearly done[498], confounded the subsequent inflammation with mere taste in the act of chewing or swallowing. At all events it is absolutely certain that the great majority of people who have been poisoned with arsenic remarked in taking it either no taste at all, or merely a roughness owing to the gritty condition of its powder.
The oxide of arsenic when subjected to heat is sublimed at 380°, or, according to Dr. Mitchell, 425° F.[499] and condenses in the form of a crystalline powder, which, if the operation is performed slowly and on a small quantity proportioned to the size of the tube, evidently consists of little, adamantine octaedres.—When it is mixed with carbonaceous matter and heated, it is reduced, and the metal is sublimed. This constitutes the test of reduction, which, when conducted with due care, may be rendered singly a certain proof of the presence of arsenic.
Water dissolves it. Its solubility is a point of some medico-legal importance; for a doubt may arise whether the quantity of a solution that has been swallowed contained a sufficient dose to cause severe symptoms or death. Different statements have gone forth on this head. Klaproth found, that a thousand parts of temperate water take up only two parts and a half,—and that a thousand parts of boiling water take up 77·75 parts or a thirteenth, and retain on cooling 30 parts or a thirty-third of their weight.[500] Guibourt found a difference between the transparent and opaque varieties; for a thousand parts of temperate water dissolved in thirty-six hours 9·6 of the transparent, 12·5 of the opaque variety; and the same quantity of boiling water dissolved of the transparent variety 97 parts, retaining 18 when cooled, but of the opaque variety took up 115 and retained on cooling 29.[501] More lately Mr. Alfred Taylor observed that temperate water, simply poured on the opaque oxide and left for seventy-two hours, contained one grain in a thousand, but if often agitated, 8·5 grains; that boiling water, occasionally agitated for the same period, contained 9·27 or 9·54 grains; that water, boiling gently for an hour dissolved 31·5, and on cooling and resting for three days retained 17; that with violent ebullition for an hour, it took up 46·3, and retained 24·7 grains on cooling and resting for three days; that a saturated boiling solution after six months contained 24 or 26 grains; and that a saturated boiling solution of the transparent oxide contained 46 or 47·5 grains, and on cooling and resting for two days retained 18·7 or 13·4 grains.[502] It is impossible to account for these discrepancies; for all the experimentalists conducted their investigations with care, and with a view to the medico-legal question stated above. Hahnemann farther remarked, that at the temperature of the blood a thousand parts of water dissolve ten parts with the aid of ten minutes’ agitation;[503] and Navier, that boiling water kept for an hour on it, and decanted off in the way an infusion is usually made, dissolves 12·5 grains in every thousand.[504]
Its solubility is impaired by the presence of organic principles. When mixed with mucus or milk it dissolves, according to Hahnemann, with great difficulty; and I have found that a cup of tea, left beside the fire at a temperature of 200° for half an hour upon two grains of the oxide, does not take up entirely even that small quantity. An important consequence of the fact now mentioned is, that when swallowed in the solid state, little or no arsenic may be found in the fluid contents of the stomach. In a case which occurred to Scheele three grains of solid arsenic were found in the contents, but hardly a trace in solution.[505] It would be wrong, however, to suppose that it is never found in the fluid contents. For, not to mention the observations of others, I have myself often detected it in the fluid part of the stomach in persons poisoned by arsenic.
The solution of oxide of arsenic in boiling water yields minute crystals on cooling, which, when their form is defined, are octaedres. In this state, on account of its whiteness and brilliancy, it exceedingly resembles pounded sugar. By spontaneous evaporation I have procured in twelve months fine octaedres nearly as large as peas. These do not become opaque by keeping, like the sublimed masses.
A difference of opinion prevails as to the action of the oxide on vegetable colours. This is a matter of no great consequence to the medical jurist; but it is right not to leave a disputed point without some notice. Guibourt says the transparent variety faintly reddens litmus, while the opaque variety faintly restores to blue litmus previously reddened.[506] My own experiments are at variance with these statements: I have always found that the solution of the powder, which is of the opaque variety, faintly reddens litmus, and does not alter reddened litmus.
The remaining chemical properties of the oxide, which it is necessary for the medical jurist to know, will be mentioned under what is now to be said of the principal test by which its presence may be ascertained. Under this head will be noticed, first the tests for the solid oxide, secondly, those for its solution, and lastly, the method of detecting it when mingled with vegetable or animal solids and fluids, such as the contents and tissues of the stomach.
The most characteristic and simple test for oxide of arsenic in its solid state, either pure or mixed or combined with inorganic substances, is its reduction to the metallic state.
Various methods have been at different times proposed for employing the test of reduction. In the ruder periods of analytic chemistry we find Hahnemann recommending a retort as the fittest instrument, and stating ten grains as the least quantity he could detect.[507] Afterwards Dr. Black substituted a small glass tube, coated with clay and heated in a choffer; and in this way he could discover a single grain.[508] In a paper published in the Edinburgh Medical and Surgical Journal, I showed how to detect a sixteenth of a grain; and afterwards even so minute a quantity as a hundreth part of a grain.[509]
The process is performed in a glass tube; which, when the quantity of the oxide is very small, should not exceed an eighth of an inch in diameter, and may be conveniently used of the form first recommended by Berzelius, and represented in Fig. 3.—The best material for reducing the oxide is recently ignited charcoal, if the quantity of suspected substance be very small. For when any of the ordinary alkaline fluxes is used, more than half of the arsenic is retained, probably in the form of an arseniuret of the alkaline metalloid. But when the quantity of matter for analysis is considerable, charcoal is inconvenient, as it is apt to be projected up the tube on the application of heat; and an alkaline flux is on that account preferable. For this purpose soda-flux,—made by grinding crystals of carbonate of soda with an eighth of their weight of charcoal, and then heating the mixture gradually to redness, so as to drive off all water,—is better than the more familiar black flux, which contains carbonate of potash; because the latter attracts much moisture when kept for some time.—If the quantity operated on is large it should be mixed with the flux before being introduced into the tube; if it is small, it may be dropped into the tube and covered with charcoal. The materials are to be introduced along a little triangular gutter of stiff paper, if the tube is large; but with a small tube it is preferable to use the little glass funnel represented in Fig. 2, to which a wire is previously fitted, for pushing the matter down when it adheres. The material should not be closely impacted. Heat is best applied with the spirit-lamp, first to the upper part of the material, with a small flame, and then to the bottom of the tube, the flame being previously enlarged. A little water, disengaged in the first instance, should be removed with a roll of filtering paper, before a sufficient heat is applied to sublime the metal. As soon as the dark crust begins to form, the tube should be held steady in the same part of the flame. With these precautions a well defined crust will be procured with facility.
The characters of the crust have been mentioned already under the head of fly-powder (p. 199). They are distinct even in crusts weighing only a 300th of a grain. A crust of this weight, a tenth of an inch broad and four times as long, may show characteristically all the physical characters of an arsenical sublimate a hundred times larger.
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.
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.
Ammoniacal nitrate of silver is prepared by precipitating the oxide of silver by means of ammonia, from a solution of nitrate of silver or lunar caustic in ten parts of water, and then redissolving the precipitate nearly, but not entirely, by adding gradually an excess of ammonia. When thus prepared, it causes, even in a very diluted solution of the oxide of arsenic, a lively lemon-yellow precipitate of arsenite of silver; which passes to dark brown under exposure to the light.—The action of this test is prevented by nitric, acetic, citric, or tartaric acid in excess, particularly by the first and last. It is also prevented by an excess of ammonia; and in very diluted solutions by the nitrate of ammonia. These facts will suggest the necessity of certain obvious precautions. Its action is obscured by the co-existence of various salts, which singly cause a white precipitate with nitrate of silver; for the yellow colour is then much lessened in intensity. The only one of these requiring special notice, because it occurs in very many of the fluids which are likely to be subjected to the researches of the medical jurist, is common sea-salt, the chloride of sodium. The best way of getting rid of the difficulty is to use in the first instance, not the ammoniacal nitrate, but the simple nitrate of silver, as long as any white precipitate falls down, to add a slight excess of that test, and then, after subsidence, to drop in ammonia. No arsenic is thrown down by the first steps of this process; but if any be present, it is subsequently thrown down in the form of the yellow arsenite of silver, on the addition of ammonia. This simple mode of getting rid of chloride of sodium was first proposed by Dr. Marcet.[519]—Ammoniacal nitrate of silver is of no use as a test for a moderately diluted solution of the oxide of arsenic, if vegetable or animal matter be present; either the colour of the precipitate is essentially altered, or no precipitate is formed at all.[520]
If the presence of arsenic is to be inferred only when the full lemon-yellow colour of the precipitate is developed, this test is not liable to any material fallacy. The presence of a phosphate, a serious obstacle according to an old way of using the silver test, is not a source of fallacy in the instance of the ammoniacal nitrate; for the yellow phosphate of silver is so soluble in the ammonia of the test, that it is not thrown down unless the phosphatic solution is very strong.—The silver test, which is extremely delicate, was proposed by Mr. Hume, a chemist of London; and in its improved state was suggested by the late Dr. Marcet. Various foreign authors have fallen into the error of supposing that nitrate of silver without an alkali precipitates oxide of arsenic: without an alkali, pure nitrate of silver gives no precipitate, or at most a bluish-white or yellowish-white haze when both solutions are strong.
Ammoniacal sulphate of copper is prepared by the same process with the last test, sulphate of copper being substituted for nitrate of silver. It is a test of very great delicacy. It causes in solutions of the oxide of arsenic an apple-green or grass-green precipitate of the arsenite of copper. The particular tint is altered apparently by trifling circumstances; but after the precipitate has stood some hours it always assumes a tint intermediate between apple-green and grass-green. The operation of this test is prevented by hydrochloric, nitric, sulphuric, acetic, citric, and tartaric acids in excess; and also by an excess of ammonia. These difficulties are obviated by manifest precautions. It is also prevented, according to Hünefeld, by muriate, nitrate, and sulphate of ammonia;[521] and by almost all vegetable infusions and animal fluids, when the oxide of arsenic is not abundant: these difficulties cannot be obviated. Even when not prevented by such fluids, its operation is often obscured, the precipitate not possessing its characteristic colour.
Ammoniacal sulphate of copper is more open to fallacies than the silver test. Of these the most important is that in some organic fluids it strikes a green precipitate, like the arsenite of copper, though arsenic be not present.[522] The solution of bichromate of potass is turned green but not precipitated by it.
On reviewing all that has now been stated regarding the liquid tests for arsenic, it will appear that there is no single test on which absolute reliance can be placed; but that the fallacies to which they are liable are generally remote, and each of them applicable to one test only. Hence if each of the three reagents, applied with due care, gives a precipitate of the characteristic tint, the proof of the presence of arsenic is decisive.
This particular view of the indications of the liquid tests, however obvious it may seem, has been often overlooked by the numerous chemists and medical jurists who have written for and against them. The antagonists of the tests have been content with proving how so many fallacies lie in the way of each, that no dependence can be put in any one of them: They have not considered that the fallacies attached to one are obviated by the conjunct indications of the others.
I am of opinion therefore that the analysis for arsenic by liquid reagents has been unjustly neglected in the present day. It is an exceedingly convenient method, and one of extreme delicacy, because by using small tubes it is easy to operate with precision on very minute portions of a suspected fluid. It is also perfectly conclusive, so far as chemical knowledge now goes. On a remarkable trial a few years ago in this country, a distinguished chemist, who, as witness for the prisoner, was made by counsel to throw discredit on the liquid tests individually, nevertheless admitted to the counsel for the prosecution, that no other substance in nature but arsenic could produce the same effects as it with the whole three tests in succession.
Reduction process.—The process by reduction of arsenic to the metallic state, as applied to the poison in a state of solution, consists in separating the whole arsenic by a liquid test in such a state as to admit of the precipitated compound being subjected to the process of reduction and sublimation. The best method of the kind is a modification of one described by me in 1824.[523] This consists in throwing down the whole arsenic in the form of sulphuret by means of hydrosulphuric acid, converting the sulphuret by the process of reduction to the metallic state, and oxidating the metal thus procured. The hydrosulphuric acid is preferred to other liquid reagents, because the precipitate it forms, while possessing a very characteristic colour, is also more bulky than those caused by the other tests, and is therefore more easily collected,—and because its action is not liable to be prevented or obscured by so many disturbing causes. The steps of the process are the following:—
The fluid to be examined must be acidulated with acetic or hydrochloric acid. If the fluid be neutral or alkaline, the acid may be added at once. If on the other hand the fluid redden litmus, and the acid be either unknown or a mineral acid, potash must first be added in a slight excess, and then the alkali must be supersaturated with acetic or hydrochloric acid. The reasons for these precautions are stated under hydrosulphuric acid as a liquid reagent. The fluid being thus prepared, it is subjected to a stream of hydrosulphuric acid gas for ten or fifteen minutes. The first portions of the gas turn the arsenical solution to a bright lemon-yellow colour, and the subsequent portions throw down a yellow flocculent sulphuret of arsenic. If the proportion of oxide in solution is small, a yellowness or yellow milkiness only is caused, owing to the sulphuret being soluble in an excess of hydrosulphuric acid. But on expelling that excess by boiling, a distinct precipitate and colourless fluid are produced. The precipitate is then to be collected thus. The precipitate is allowed to subside, and the supernatant fluid being withdrawn, the remainder is poured into a filter. When all the fluid has passed through, the portions of precipitate on the upper part of the filter are washed down to the bottom. The filter is then gently compressed between folds of bibulous paper, and the sulphuret removed with the point of a knife before it dries, and dried in little masses on a watch-glass by the side of a chamber-fire, or still better in a vapour-bath. In this way it is very easy to collect a twenty-fifth part of a grain of the sulphuret. Another method which takes more time, but will enable the least skilful person to collect extremely small quantities, is to allow the sulphuret to subside in the original fluid in which it is formed, to pour off the supernatant liquid, and pour the remainder into a small glass tube, Fig. 7. After the precipitate has thoroughly subsided, the supernatant liquid is to be withdrawn, and its place filled up with boiling water. The operation of alternate subsidence and affusion being repeated a sufficient number of times, the last portions of water should be gently driven off by heat, and wiped off the inside of the tube as the drops condense on it. Finally, the bottom of the tube, with the precipitate attached, is to be cut away with the file, and broken into small fragments with the view of preserving the whole sulphuret for the process of reduction. The sulphuret having been collected in either of these ways, it is now to be dropt into the tube, Fig. 3, and covered by means of the funnel, Fig. 4, with soda-flux. The process in other particulars is the same with that for reducing solid oxide of arsenic.
This method of investigation gives extremely precise results, because it presents the poison successively in three distinct forms, as sulphuret, metal, and crystallized oxide, all of which possess very prominent and characteristic external properties. It is also a method which is capable of detecting very minute quantities of oxide of arsenic. And it has the advantage over the process by liquid reagents of being applicable to organic fluids. It was accordingly followed in most medico-legal researches until the recent discovery of the methods of Marsh and Reinsch.
In order to render it quite satisfactory, it is necessary to go through the steps of the analysis at the same time with distilled water, lest any of the reagents used should accidentally contain arsenic.
Process of Marsh.—This method consists in disengaging arsenic from the solution in the form of arseniuretted-hydrogen gas, burning the gas in such way as to obtain either metallic arsenic or oxide of arsenic, and subjecting the product to various tests.
I have called this beautiful method of analysis Marsh’s process, because it appears to me that injustice has been done its discoverer both by himself and those who have since investigated the subject, when they denominated it merely a test. Medico-legal analysis stood in no need of a new test for arsenic, but very much of an easy and infallible method of detaching minute quantities of it in a state of purity from simple and compound fluids, so as to admit of its being accurately examined. It is this important object, and not strictly speaking a new test, that has been attained through means of the discovery of Mr. Marsh.
His discovery consists in the observation, that, if hydrogen gas be disengaged by the action of sulphuric acid or zinc in a fluid containing arsenic dissolved in any form, arseniuretted-hydrogen gas is disengaged along with the hydrogen; and that if the two gases be burnt together in a fine flame, metallic arsenic is deposited on a white porcelain surface held in the flame, and oxide of arsenic if the porcelain be held immediately above it.[524] The production of a brilliant mirror-like crust in the former case, and of a white powdery one in the other, constituted Marsh’s test as originally proposed; and it was at first conceived to furnish unimpeachable evidence of the detection of arsenic. Afterwards many inquirers, and among them the discoverer himself, became satisfied that certain fallacies stand in the way of a conclusion based on such simple premises. Various supplementary tests were in consequence proposed. And at length it seems to be agreed, that the proper mode of applying Marsh’s discovery is to employ a succession of tests, of which that originally pointed out by him is the first. A vast variety of methods of analysis founded on this principle have been proposed by British and continental chemists. It would be tedious and unprofitable to discuss or even to state them here. The reader will probably be satisfied with a reference to the most important of them[525] and with a description of that process, which appears to me, from repeated trials in medico-legal practice, to be at once most convenient, delicate, and conclusive.
Let the liquid to be examined be introduced into a Döbereiner’s lamp [Fig. 10], or an apparatus constructed with a bottle and a funnel upon the same principle [Fig. 11]; and dilute the liquid with distilled water, until the lower cavity of the apparatus be nearly full, leaving space however for the tube of the funnel, a fragment of zinc, and some sulphuric acid. Put in a cylinder or rod of zinc, a; and then add sulphuric acid until a moderate effervescence ensue. Close the junction of the two vessels, and then, allowing a little gas to escape at c, shut the stop-cock, and let the gas fill the vessel A, by driving the liquid up into B. Having meanwhile fitted by a cork to the exit-tube, c, the glass tube, d e, which is loosely stuffed with raw cotton at the end d g, and has a bent plate of copper or tinned iron hung over it at f,—open the stop-cock, allow a little gas to escape so as to expel the air in d e, and then kindle the gas at e, which must be contracted to a capillary opening. Keep the flame low, and hold the surface of a white porcelain vessel across the middle of it for a few seconds. If no stain be produced on the porcelain, there is no arsenic in the fluid. If a stain be formed, regulate the escape of gas by the stop-cock so that the fluid may not rise above the middle of the lower vessel of the apparatus, and apply the heat of a spirit-lamp flame to the tube d e on the left hand of the plate f, the purpose of which is to prevent the heat being communicated beyond that point. By and by, if there be arsenic in the fluid, a brilliant metallic ring will appear beyond f, owing to decomposition of arseniuretted-hydrogen gas. As soon as the crust is thick enough to present its properties characteristically, withdraw the spirit-lamp; place the tube e h so that the flame at e shall be completely within the ball, i; let the tube incline very slightly in the direction from k to l; and allow a stream of cold water to trickle down upon the portion k l, which should be wrapped in a single layer of calico. Oxide of arsenic will gradually condense, partly in white powder or minute sparkling crystals in the ball and between i and k, and partly between k and l in the form of a solution, which collects at the bend l. The solution which may be increased in quantity by boiling a little distilled water upon the powder in the ball and bend i k, is then to be subjected in small portions to the three liquid reagents, ammoniacal nitrate of silver, ammoniacal sulphate of copper, and hydrosulphuric acid.
Some experience is required to apply this process successfully. But with due attention it furnishes conclusive evidence with great delicacy and precision. A solution containing only a millionth part of oxide of arsenic will part with it readily in the form of arseniuretted-hydrogen; and the slightest trace of that gas in the hydrogen is indicated by the method recommended above.—The process is compounded of Mr. Marsh’s original discovery, the supplementary test of reduction in the exit-tube recommended by Berzelius,[526] and the formation and examination of the oxide proposed by myself.[527]—With certain precautions and modes of manipulating, it is applicable to the most complex organic fluids, as well as to simple solutions.
The discovery of Mr. Marsh had not been long made before the test in its original simple form was found liable to divers important fallacies. It appeared, for example, that antimony yields very nearly the same appearance of metallic crust and of white powder, according to the position of the porcelain in the flame; that some porcelains glazed with oxide of zinc are similarly stained by a flame of simple hydrogen gas; that a great variety of metallic salts, if spirted up into the exit-tube, undergo reduction in the flame, and cause imitative stains on the porcelain; that iron-salts seems to form stains from the same chemical action as what occurs in the case of arsenic; and that certain compounds of phosphorous acid with ammonia and animal matter, or even mere animal matters themselves, will in some circumstances produce a stain more or less similar to that which is occasioned by arsenic.
There is no doubt, that the resemblance of most of these spurious stains to an arsenical crust has been much exaggerated. But still the similarity is sufficient to satisfy every impartial judge, that the mere production of a brilliant metallic, or white powdery stain, or both, upon porcelain, is not conclusive evidence of the detection of arsenic in medico-legal inquiries. It is strong presumptive evidence; and the non-production of such stains is absolute proof that arsenic is not present. But in order to obtain irrefragable proof of its presence, the substance which forms the crusts and stains must be subjected to farther examination. And such is the object of the supplementary methods in the process detailed above. That process is perfectly free of fallacy. No substance yet known but arsenic can yield the succession of phenomena which have been detailed. My opinion farther is, that the process may be safely simplified by withdrawing Berzelius’s supplementary test of reduction in the exit-tube, and retaining the test of oxidation only, with the examination of the oxide by liquid reagents. I have retained the former in deference to the opinion expressed by a committee appointed by the French Institute, who examined the whole subject with unwearied zeal, but who, it may be observed, seem never to have had in their view the check-test of oxidation; which, with the consecutive tests, is superior in conclusiveness to the check of reduction only.
Reinsch’s process, like the former, has been inconveniently called a new test for arsenic. The fact discovered by Dr. Reinsch is valueless as supplying a mere test; but it forms the ground-work of the best process of all yet proposed for the detection of arsenic in solution. The discovery is, that arsenic in solution is deposited in the metallic state upon copper-leaf, when the fluid is acidulated with hydrochloric acid, and heated till it boils gently or is about to do so; and that by heating the copper gently in a glass tube the arsenic is sublimed from it in the form of oxide or metal according to the quantity present.[528]
This method is so simple and easy as scarcely to require any detailed explanation. The fluid should contain about a tenth of its volume of hydrochloric acid. It must be heated near ebullition before the copper is introduced, otherwise the copper becomes tarnished, though arsenic be not present. Copper-leaf, or copper-plate worn thin by the action of diluted nitric acid, or fine copper gauze, is the best form for use. In the feeblest solutions ten or fifteen minutes elapse before arsenic is visibly deposited, and forty minutes should be allowed for strong deposition; but in strong solutions, the action takes place in a few seconds. The result is a thin, brittle brilliant, steel like coating of metallic arsenic. As soon as the deposit is formed, the copper is to be removed, dried with a gentle heat, cut into small shreds, and heated with a spirit-lamp in the smallest glass tube that will conveniently contain the whole; upon which a metallic ring of arsenic is sometimes sublimed, but more generally a ring of small sparkling crystals. These are first to be examined as to their form with a common pocket lens; and then dissolved in boiling distilled water, after shaking out the copper, so that a solution may be obtained and subjected to the liquid reagents, especially the ammoniacal nitrate of silver as being the readiest and most delicate. In all medico-legal inquiries it is necessary to perform a preliminary experiment with distilled water and the hydrochloric acid used, lest the acid contain arsenic.
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.
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.
Process by Hydrosulphuric Acid.—This method may be employed in two ways, according as the object is merely to prove the presence of oxide of arsenic, or to ascertain also its quantity.
a. If proof of its presence be all that is wanted, cut any soft solids into small pieces, add distilled water if necessary, boil for half an hour, let the decoction cool, and filter it. Add a little acetic acid to the filtered fluid, and if any precipitate form, filter again. Evaporate to dryness, first by ebullition, afterwards over the vapour-bath. Dissolve the residuum again in repeated portions of boiling distilled water, and filter the solution. If it be not acid to litmus-paper add more acetic acid, and transmit hydrosulphuric acid gas through the fluid until an excess be indicated by the sense of smell after agitation, Then expel the excess of gas by boiling; and if the precipitate of sulphuret of arsenic do not subside readily add a little of a strong solution of hydrochlorate of ammonia, which will facilitate subsidence. When the precipitate has fallen to the bottom, withdraw the supernatant fluid with the pipette, Fig. 8; and replace it with a little boiling distilled water. Lastly, collect the precipitate on a filter, and proceed as by the reduction process with soda-flux for oxide of arsenic, in a state of simple solution.
This method answers very well for ordinary cases where the quantity of arsenic is not extremely minute. But I have met with instances in medico-legal practice where the process of Reinsch, as well as that of Marsh, succeeded in detecting the poison in sources to which the method by hydrosulphuric acid had been applied without avail; because apparently the organic matter existing in solution prevented the action of the gas, or, as Orfila thinks, because boiling water will not in all circumstances remove oxide of arsenic from the textures of the animal body which are impregnated with it. In particular I doubt whether this method is sufficiently delicate to detect arsenic in those organs and textures into which it has been conveyed in cases of poisoning through absorption into the blood.—Another objection is its tediousness. The first filtration, if the substance to be examined be the stomach or its contents, may take two days; and one way or another the analysis can seldom be completed within four days. Reinsch’s process may be brought to a conclusion in two hours or less, even in the most difficult circumstances.
b. The last process to be mentioned, is one based, like the previous one, upon the precipitation of arsenic in the form of sulphuret, but with very material modifications, the purpose of which is to enable the analyst to separate the whole arsenic in a state of purity, so as to ascertain the exact amount of the poison in the mixture. This method has been recently proposed by Drs. Fresenius and von Babo.[534]
Cut any soft solids into small pieces, put the whole into a porcelain basin, add as much hydrochloric acid as equals the probable weight of the dry matter in the mixture, and then water enough to form a thin pulp. Heat the basin over the vapour-bath, adding every five minutes about half a drachm of chlorate of potass, and stirring frequently, until the liquid become clear-yellow, homogeneous, and thin. Add now two drachms more of the chlorate; filter through linen, washing the residuum on the filter with boiling water; concentrate to a pound; add a strong solution of sulphurous acid till its odour predominates, and expel the excess of it by heat. The liquid is now ready for the transmission of hydrosulphuric acid gas, which should be transmitted in a slow stream for twelve hours. Wash away any sulphuret adhering to the tube by means of ammonia, and add the solution to the principal liquid; which is next to be left at a gentle heat about 80° F., in a vessel covered with paper, till the sulphureous smell entirely disappear. The precipitate, which contains organic matter as well as sulphuret, is then to be collected on a paper filter, washed, and dried with the filter over the vapour-bath. The animal matter is next destroyed, and the sulphuret converted into arsenic acid, by dropping on it fuming nitrous acid till the whole is moistened, drying the product thoroughly over the vapour-bath, moistening the residuum with concentrated sulphuric acid, heating the mixture again in the vapour-bath for two or three hours, and raising the heat afterwards gradually in a sand-bath to 300° F., till a charred brittle mass be obtained. This is to be heated over the vapour-bath with twenty parts of distilled water, filtered, and washed with boiling water on the filter till what passes through ceases to redden litmus. The solution, which ought to be colourless, is next acidulated with hydrochloric acid, and treated as formerly with hydrosulphuric acid gas. When the sulphuret has been collected on a small filter, diluted ammonia is to be sent through the filter as long as it dissolves any sulphuret, and is to be received in a weighed porcelain basin, in which the ammonia and water are to be driven off at a temperature not exceeding 212°. The sulphuret which is alone left may now be weighed by again weighing the basin; and one grain of sulphuret is equivalent to 0·803 of a grain of oxide of arsenic.—The authors add an elaborate process for obtaining from this the whole arsenic by reduction. But such a proceeding is unnecessary. It is sufficient in medico-legal inquiries to ascertain by the simpler method given above [p. 204], that it does yield by reduction with soda-flux a true arsenical crust, and that this yields by oxidation white, sparkling crystals with triangular facettes.
After a comparative trial of the most esteemed process, Drs. Fresenius and von Babo state that they found the one now described as delicate as any other, and the only method by which the quantity of oxide of arsenic can be ascertained with accuracy.—The hydrochloric acid used at the commencement enables the water to dissolve compounds of arsenic which water alone will not act on; and it farther facilitates solution by breaking up or dissolving organic textures. The addition of chlorate of potash prevents the escape of oxide of arsenic during the subsequent evaporation; which is apt to happen when hydrochloric acid is present. The subsequent addition of sulphuric acid converts arsenic acid into arsenious acid, in which shape the sulphuret of arsenic is more readily formed by the action of hydrosulphuric acid gas, when organic matter co-exists in the solution. The steps for destroying organic matter thrown down with the sulphuret at its first formation require no further commentary: They are the most important particulars in the process for its main object,—the determination of the quantity of pure sesqui-sulphuret, and, through it, of the sesquioxide originally in the subject of analysis.
Before taking leave of the detection of arsenic in organic mixtures, it is necessary to notice certain alleged fallacies in the way of every process, arising from arsenic obtaining admission into the subject of analysis through other means than its intentional addition or its introduction as a poison into the body. This topic, one of paramount importance in medico-legal chemistry, has lately undergone careful investigation during and since the notorious trial of Madame Lafarge. The results are the following:—
It has been alleged that arsenic may obtain accidental admission into the subject of analysis, 1, because the reagents used in the processes may be adulterated with arsenic; 2, because the material of the apparatus may contain it; 3, because it may have existed in antidotes administered during life; 4, because it sometimes forms a constituent part of the human body in the natural state; and 5, because it exists in the soil of some churchyards.
1. Arsenic may exist as an adulteration in some reagents.—It must be apt to occur in sulphuric acid, when that substance is prepared with pyritic sulphur, which commonly contains some sulphuret of arsenic; and it has actually been found in abundance in the acid by various experimentalists, and in England for the first time by Dr. Rees.[535] It may be detected by transmitting hydrosulphuric acid gas through the diluted acid; and it may be effectually removed in the same way,[536] the acid being afterwards filtered in a funnel whose throat is filled with asbestus, and the excess of gas being expelled by heat.—Hydrochloric acid may contain arsenic, because it may have been prepared with an arsenicated sulphuric acid. The impurity may be detected and removed in the same way as in that substance. Nitric acid seems not apt to be similarly adulterated;[537] but it may be tested by Marsh’s process, after neutralizing the acid with potash, and adding more sulphuric acid than is required to decompose the nitre thus formed. Zinc occasionally contains a little arsenic, which will be evolved in Marsh’s process. Dr. Clark of Aberdeen says zinc is scarcely ever free of a trace of arsenic; and it has been occasionally detected by others. Orfila, however, very seldom found so much as to be discoverable by Marsh’s test applied continuously for a great length of time.[538] A committee of the French Institute came to the same conclusion.[539] M. Jaquelain, acting under the directions of Professor Dumas, could not detect an atom in any French specimen of zinc, or its carbonate or silicated oxide, as met with in commerce.[540] Lastly, Mr. Brett satisfied himself that no British or foreign zinc he could obtain indicated the presence of arsenic by a process capable of detecting a 5000th of that metal in zinc.[541] It is an obvious inference from all these inquiries that no difficulty can be experienced in obtaining zinc so pure as to exhibit not a trace of arsenic by Marsh’s method. Neither is there any difficulty in obtaining sulphuric, muriatic, and nitric acid free of that adulteration.
But at the same time it is equally obvious, that in medico-legal analyses, unless the reagents used be previously known to be free of arsenic, they ought invariably to be subjected in the first instance to the process, whatever it may be, which the analyst proposes to employ for detecting arsenic in a suspected substance.
2. Arsenic may be present in some articles of chemical apparatus.—Arsenic has been detected in the metal of cast-iron pots,[542] which Orfila and others have proposed to employ in certain analyses on the large scale, as, for example, when the poison is sought for in the whole soft solids of the human body. It is denied, however, that any of that arsenic can be dissolved out of cast-iron by the process which has been followed in such circumstances.[543]
The primary fact, and the qualification of it, are in my opinion of equally little medico-legal importance. It is not likely that such enormous masses of material will ever be operated on again, as those which were made use of in some late, French trials, and for which great iron pots were found indispensable;—because it has been proved that absorbed arsenic is chiefly to be met with in particular organs or secretions, such as the liver and urine. Besides, a false importance has been attached to the enthusiastic analyses of the whole human carcase, with which some French chemists have been astounding the minds of the scientific world, as well as the vulgar, on the occasion of certain late trials for poisoning. I confess I could not find fault with a jury, who might decline to put faith in the evidence of poisoning with arsenic, when the analyst, after boiling an entire body, with many gallons of water, in a huge iron cauldron, making use of whole pounds of sulphuric acid, nitric acid, and nitre, and toiling for days and weeks at the process, could do no more than produce minute traces of the poison. What man of common sense will believe, that, with such bulky materials and crude apparatus, it is possible to guard to a certainty against the accidental admission of a little arsenic? At all events I am much mistaken if any British jury would condemn a prisoner on such evidence,—or any British chemist find fault with them for declining to do so.
3. Arsenic may have existed in antidotes administered during life.—It is now generally known, that the only chemical antidote for arsenic is the hydrated sesquioxide of iron. But this substance appears occasionally to contain a little arsenic, obviously derived from the compound of iron whence the oxide is prepared.[544] Such an adulteration must be rare in what is prepared by the ordinary processes, according to which the oxide of arsenic ought to remain in solution. The only effectual mode, however, of guarding against this source of error, when the antidote has been administered, is to examine a portion of the stock whence the patient was supplied, by dissolving it in an excess of sulphuric acid, and subjecting it to Marsh’s test.
4. Arsenic sometimes exists naturally in the human body.—This startling proposition was first advanced by M. Couerbe, and by Professor Orfila soon afterwards.[545] The latter subsequently stated, that it exists only in the bones, and not in any of the soft solids.[546] It is now clear, however, that both of these experimentalists must have committed an error. Orfila himself admits that his early researches are vitiated by the subsequent discovery of arsenic in some kinds of sulphuric acid;[547] and all recent attempts by others to obtain his results have failed. Thus MM. Flandin and Danger could not detect arsenic in any part of the human body, when it had not been administered:[548] Pfaff was unable to detect an atom of it in the bones of man or the lower animals by Orfila’s own process:[549] Dr. Rees was equally unsuccessful:[550] and in 1841 a committee of the French Institute, who superintended the performance of an analysis in three cases by Orfila, reported that he failed in every instance to find a trace of arsenic, by a process which could detect a 65th part of a grain intentionally mixed with an avoirdupois pound of bones.[551]
There is the strongest possible presumption, therefore, that human bones never contain any arsenic. And besides, supposing they did, the source of fallacy would be utterly insignificant; for, when it becomes necessary to search for arsenic absorbed into the textures of the body, it is never necessary to have recourse to the bones.
5. Arsenic may exist in the soil of churchyards.—This proposition too was first announced by Professor Orfila, who found a little in the churchyard of Villey-sur-Tille, near Dijon, and of the Bicêtre, Mont-Parnasse, and New Botanic Garden at Paris.[552] And although MM. Flandin and Danger afterwards denied they could ever find any,[553] a committee of the Parisian Academy of Medicine reported that Orfila proved before them the accuracy of his statement.[554] But the arsenic exists in a state in which it cannot be dissolved out by boiling water: It has been hitherto separable only by boiling the churchyard mould with concentrated sulphuric acid. Hence it cannot pass by percolation through a coffin into a body; and consequently it becomes a source of fallacy only when the coffin has been broken up in the course of time, and the mould lies in actual contact with the organs to be analysed.[555]