LAKES

Under this title are comprised all those colours which consist of a vegetable dye, combined by precipitation with a white earthy basis, which is usually alumina. The general method of preparation is to add to the coloured infusion a solution of common alum, or rather a solution of alum saturated with potash, especially when the infusion has been made with the aid of acids. At first only a slight precipitate falls, consisting of alumina and the colouring matter; but on adding potash, a copious precipitation ensues, of the alumina a ociated with the dye. When the dyes are not injured, but are rather brightened by alkalis, the above proce is reversed; a decoction of the dye-stuff is made with an alkaline liquor, and when it is filtered, a solution of alum is poured into it. The third method is practicable only with substances having a great affinity for sub sulphate of alumina; it consists in agitating recently precipitated alumina with the decoction of the dye.