Raspail’s Method
M. Raspail suggests the following method for detecting arsenic: The surface of a brass plate is rasped by filing. In this condition the plate may be regarded as an innumerable quantity of voltaic elements, formed by the juxtaposition of the molecules of zinc and copper. The suspected materials are boiled with caustic potassa, the solution filtered, a drop of the filtrate placed upon the brass plate, and a drop of chlorine water added. If the plate is then allowed to stand for a moment and the substance under examination contains arsenic, a mirror-like spot is soon deposited upon its surface. In order to avoid confounding this deposit with those produced by other metals, the substitution of granulated brass for the plate is in some cases advisable. The granulated metal is dipped successively in the suspected solution and in chlorine water. The granules retain a small quantity of the solutions and, owing to the action of the chlorine water, become covered with metallic spots, if arsenic be present. They are then dried, placed in a tube closed at one end, and exposed to the heat of an alcohol lamp. In case the spots are arsenical, the metal volatilizes and condenses in a ring upon the cold part of the tube, which is submitted to the tests previously described.
This method can hardly be of great service, inasmuch as it extracts the poison from but a very small portion of the solution containing it: we have not, however, personally tested its merits.G