This pigment, which is known in the market as “pink colour,” may be used with advantage in oil painting in the place of rose madder lake, over which it has the advantage in durability. =Silver Chromate.=—A solution of silver nitrate gives, with potassium chromate, a deep red precipitate of silver chromate, which is occasionally used as an artists’ pigment under the name of “purple red”. Not only is the price of this pigment high, but it has little durability, since, like all silver compounds, it is acted upon by sulphuretted hydrogen and turned dark, owing to the formation of black silver sulphide. CHAPTER XXI. PURPLE OF CASSIUS. Purple of Cassius, or gold purple, is not a pigment for painters’ use; it is employed for colouring glass, for porcelain painting and for coloured glazes. For these last purposes we know nothing which can completely replace purple of Cassius, so that its preparation will be described here. Its costliness prevents its general use. As regards the chemical constitution of purple of Cassius very diverse views are held. Chemists have not yet succeeded in explaining it; some regard it as aurous stannate, others as stannic acid, in which a particular form of gold of a red colour is contained in an extremely finely divided condition. There are reasonable grounds for both opinions. Theoretical considerations are of little interest to the manufacturer, who is chiefly interested in the method by which he can prepare a product which satisfies the demands of the consumer. Purple of Cassius has been known for nearly two hundred years, hence there are many formulas for its preparation. Before these are enumerated it is necessary to give an account of the conditions under which this substance is formed. In order to obtain gold purple, a solution is required which contains both stannous and stannic chlorides; if this solution is mixed with a very dilute solution of gold chloride a precipitate is obtained which is generally brownish red, and only acquires its fine red shade on igniting. The colour shown by the purple after precipitation is no guide to the shade of the finished product; a purple which shows a very fine colour whilst wet often produces a pigment of much less beauty than an actually ugly precipitate. These differences are to be ascribed to the varying molecular condition of the purple, for the different varieties show very small differences in chemical composition.