Besides those mentioned, there are other metallic whites varying in beauty and opacity, such as those of mercury, arsenic, and antimony; but none of them are of any value or reputation in painting, on account of their great disposition to change of colour, whether by light or foul air, both in water and oil. There are also other terrene whites, under equivocal names, among which are Morat or Modan white, Spanish white, Troys or Troy white, Rouen white, China white, and Satin white; the latter being a sulphate of lime and alumina, which dries with a glossy surface. The common oyster-shell contains a soft white in its thick part, and there is the white of egg-shells. There is, too, an endless variety of native earths, in addition to those prepared by art. The whole of them, however, are destitute of body in oil; and several, owing to their alkaline nature, are injurious to many colours in water, as well as to all colours which cannot be employed in fresco. Among the infinitude of white substances, the artist finds that there are but three white pigments--those of lead, zinc, and baryta. The first possesses the greatest opacity, while the second and third are most durable. The last, however, has so many objectionable qualities, that the number of eligible whites, may almost be said to be two--lead and zinc. Of these, the former is blackened by foul air, and in oil, the latter is wanting in body. In fact, there is but one white pigment which approaches perfection--Chinese white; and this is only a water-colour. CHAPTER VIII. ON THE PRIMARY, YELLOW. Yellow is the first of the primary or simple colours, nearest in relation to, and partaking most of the nature of, the neutral white; it is accordingly a most advancing colour, of great power in reflecting light. Compounded with the primary _red_, it constitutes the secondary _orange_, as well as its relatives, scarlet, and other warm colours. It is the archeus, or prime colour, of the tertiary _citrine_; characterises in like manner the endless number of semi-neutral colours called _brown_, and enters largely into the complex hues termed buff, bay, tawny, tan, dan, dun, drab, chestnut, roan, sorrel, hazel, auburn, isabela, fawn, feuillemort, &c. Yellow is naturally associated with red in transient and prismatic colours, and is the principal power with it in representing the effects of warmth, heat, and fire. Combined with the primary _blue_, yellow furnishes all the variety of the secondary _green_, as well as, subordinately, the tertiaries _russet_ and _olive_. It also enters in a very subdued degree into cool, semi-neutral, and broken colours, and assists in minor proportion with blue and red in the composition of _black_.