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.