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.[1] 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[2] 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 stopcock, 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
- ↑ Edinburgh New Philosophical Journal, 1836, xxi. 229.
- ↑ Mr. L. Thomson in Lond. Phil. Journal, 1837, i. 353.—Orfila, Journal de Chimie Médicale, 1841, p. 212.—Bischoff, Repertorium für die Pharmacie, lxxv. 411.—Mr. H. H. Watson, Manchester Memoirs, vi. 603.—Pettenkoffer, Repertorium für die Pharmacie, lxxvi. 289.—Berzelius, and a Committee of the French Institute, Journal de Chimie Médicale, 1841, 393.—Flandin and Danger, Ibidem, 1841, 435.—Malapert, Ibidem, 1841, 295.—Lassaigne, Ibidem, 1840, 638,—Mr. Ellis, Lancet, 1843.—A paper of my own, Edinburgh Monthly Journal of Med. Science, iii. 257.
