«Colloidal Gold.»—When a solution of gold chloride[234] is treated with a solution of stannous chloride which contains a little stannic chloride,[235] a purple-red, flocculent precipitate—the "purple of Cassius"—is formed; in extremely dilute solutions only a purple-red liquid[236] may be produced. If the precipitate is collected on a filter, washed with water and then treated, on the filter, with a few drops of ammonium hydroxide solution and some water (‹exp.›), it is seen to ‹dissolve› and a beautifully colored liquid, of purple-red or claret-red tint, is found to pass through the filter. In spite of the extreme insolubility of metallic gold, the red ammoniacal solution (as well as the first red precipitate) contains «metallic gold», in the colloidal condition, formed according to the equation[237] 2 Au^{3+} + 3 Sn^{2+} ⥂ 2 Au + 3 Sn^{4+}. By the careful [p127] reduction of gold chloride with phosphorus,[238] or with formaldehyde in dilute, slightly alkaline solution,[239] brilliant red liquids, containing metallic gold in the colloidal condition, may be prepared, which remain clear for months. The passage of an electric current, in the form of an arc, between two gold points under pure water produces similar red liquids[240] containing metallic gold. «Colloidal Silver.»—Further, although silver is likewise an extremely insoluble metal, solid preparations of silver are known which, on treatment with water, form apparently perfectly clear (opalescent) liquids or solutions (‹exp.›).[241] From these silver is not deposited, even in the course of months. If a little of the black solid is heated on the lid of a porcelain crucible, the metal may be readily recognized by its white color and luster.[242]