As a pigment, zinc white may be said to be innoxious. As oxide of zinc does not readily form a saponaceous compound with fats or oil like white lead, the paint prepared with it and ordinary linseed oil does not dry or harden so rapidly. For the purpose of causing it to be more siccative, the oil was boiled with a large quantity of litharge, but by this method the white was liable to tarnish on meeting with foul air. Instead of litharge, experiments have led to the choice of salts of zinc, such as the chloride or sulphate, a small percentage of which, on being mixed with the oil or oxide, confers upon it the property of rapidly hardening. The same result is attained by employing an oil, dried by boiling with about five per cent of peroxide of manganese. In either case, a paint retaining its white colour permanently is produced. These agents might, with advantage, be more generally used in the place of litharge for rendering oils siccative. Many pigments which are not naturally affected by sulphurous emanations are apt to suffer if mixed with an oil made drying by means of lead. * * * * * 16. _Cadmium White._ Provided the metal be freed from iron, which we have commonly found to be more or less present, a white of considerable beauty may be produced; either directly by precipitation as hydrated oxide or carbonate, or indirectly by exposing the brown anhydrous oxide to air and light--the latter mode yielding a product of greater opacity. However prepared, cadmium white is deficient in body, and apt to assume a yellow tint on meeting with an impure atmosphere. 17. _Pearl White_ Is an insoluble basic nitrate of bismuth, a pearly white powder of loose texture, turning grey on exposure, and blackened by sulphuretted hydrogen. It is chiefly used as a cosmetic, but is said to injure the skin, rendering it yellow and leather-like; and it has been known to cause a spasmodic trembling of the face, ending in paralysis. Another preparation under this name, and now obsolete we believe as a pigment, was obtained from mother-of-pearl. It is described as exquisitely white, and of good body in water, but of little force in oil or varnish. 18. _Tin White_ Resembles zinc white in some respects, but has less body and colour, and dries badly. According to its composition, it is liable to turn either black or a dull yellow in contact with sulphurous vapours. 19. _White Chalk_ Is a well-known native carbonate of lime, employed by the artist only as a crayon, or for tracing his designs, for which purpose it is sawed into suitable lengths. White crayons and tracing chalks, to be good, must work and cut free from grit. From this material are prepared whitening and lime, which form the bases of many cheap pigments and colours, used in distemper, paper-staining, &c.