Purple of Cassius, or Cassius's Purple Precipitate, was discovered in 1683 by Cassius of Leyden. It is a compound of tin and gold, best formed by mixing aqueous perchloride of iron with aqueous protochloride of tin, till the colour of the liquid has a shade of green, and then adding this liquid, drop by drop, to a solution of perchloride of gold, which is free from nitric acid and very dilute: after twenty-four hours the purple is deposited. When recently prepared, the colour is brightened by boiling nitric acid. Not brilliant, but rich and powerful, this purple varies in hue according to the mode of manufacture from deep crimson to murrey or dark purple: it also differs in degrees of transparency. Working well in water, it is an excellent though costly pigment, once popular in miniatures, but at present rarely, if ever used, as purple madder is cheaper, and perfectly well supplies its place. Retaining its colour at a high red heat, it is now confined to enamel and porcelain painting, and to tinging glass of a fine red. If, whilst in its hydrated state, it be washed with ammonia, a bright purple liquid results, from which a violet colour, somewhat less expensive, can be produced, by combining the gold purple with alumina, and calcining the product in the same way that is practised with cobalt. This compound may be exposed to the action of the sun's rays for a year without being sensibly affected. 220. _Prussian Purple._ A prussiate of iron is obtainable of a violet hue, affording good shadow tints and clear pale washes. It has not, however, been introduced as a pigment, as ordinary Prussian blue tinged with red furnishes a similar colour. 221. _Sandal Wood Purple._ Sandal wood contains about 16 per cent. of colouring matter, soluble with difficultly in water, but readily dissolved by alcohol. From the latter solution, chloride of tin throws down a purple, and sulphate of iron a deep violet precipitate; neither of which is remarkable for permanence. 222. _Tin Violet._ By heating chromate of stannic oxide to bright redness, a dark violet mass is obtained, which is better adapted to enamel painting than to the palette. It communicates in glazings a variety of tints, from rose-red to violet. * * * * * So scant is the number of good purples in common use, that there are but two which can be classed as durable, namely, purple madder and the true Mars violet. Foremost in the second group stands burnt carmine. As there are different degrees both of permanence and fugacity, so are there different degrees of semi-stability. Burnt carmine, burnt lake, Indian purple, and violet carmine, all belong to this division; but the first certainly is more permanent than the rest.