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. _Of the Tests for Oxide of Arsenic in Organic Mixtures._