_Marsh’s Test or Process._—This process is founded on the decomposition of arsenious acid by nascent hydrogen, and the formation of arsenuretted hydrogen gas, which possesses the following properties: It burns with a bluish-white flame, and white smoke (arsenious acid), possessing a slight garlic odor. If a piece of glass or porcelain be held in the flame a blackish metallic stain will be deposited upon it, consisting of metallic arsenic. This stain might be confounded with one produced by antimony under similar circumstances. But the antimonuretted hydrogen gas does not burn with the odor of the arsenuretted hydrogen; while the antimonial stain is sooty, and has not a metallic lustre. The arsenical stain is further readily dissolved by a solution of chloride of lime (bleaching powder), whilst the antimonial stain is not affected. To the stain may be added a few drops of strong nitric acid; it will dissolve the arsenic; if this be allowed to evaporate, and the acid be neutralized, a few drops of nitrate of silver solution will give a brick-red precipitate of arseniate of silver. Marsh’s test is thus accomplished: a flask furnished with a cork through which pass a funnel and a long bent tube drawn to a point, is prepared so that the funnel reaches almost to its bottom. Several pieces of pure zinc are introduced, and then some sulphuric acid is poured through the funnel. In this way hydrogen gas is produced and escapes by the bent tube. When all the air has been expelled the gas should be ignited as it escapes, and a piece of cool porcelain held over the flame. If there be no deposit it is plain that neither the zinc nor the sulphuric acid, contains arsenic. A portion of the suspected fluid is next introduced into the flask by the funnel, and the issuing gas again tested. Should a stain giving the characters alluded to above be produced, the fluid contains arsenic in some form or other.