The principal phenomena in chemical colours are produced by the oxydation of metals, and it will be seen how important this consideration is at the outset. Other facts which come into the account, and which are worthy of attention, will be examined under separate heads; in doing this we, however, expressly state that we only propose to offer some preparatory suggestions to the chemist in a very general way, without entering into the nicer chemical problems and questions, or presuming to decide on them. Our object is only to give a sketch of the mode in which, according to our conviction, the chemical theory of colours may be connected with general physics. XXXV. WHITE. 494. In treating of the dioptrical colours of the first class (155) we have already in some degree anticipated this subject. Transparent substances may be said to be in the highest class of inorganic matter. With these, colourless semi-transparence is closely connected, and white may be considered the last opaque degree of this. 495. Pure water crystallised to snow appears white, for the transparence of the separate parts makes no transparent whole. Various crystallised salts, when deprived to a certain extent of moisture, appear as a white powder. The accidentally opaque state of a pure transparent substance might be called white; thus pounded glass appears as a white powder. The cessation of a combining power, and the exhibition of the atomic quality of the substance might at the same time be taken into the account. 496. The known undecomposed earths are, in their pure state, all white. They pass to a state of transparence by natural crystallization. Silex becomes rock-crystal; argile, mica; magnesia, talc; calcareous earth and barytes appear transparent in various spars.--Note T. 497. As in the colouring of mineral bodies the metallic oxydes will often invite our attention, we observe, in conclusion, that metals, when slightly oxydated, at first appear white, as lead is converted to white lead by vegetable acid. XXXVI. BLACK. 498. Black is not exhibited in so elementary a state as white. We meet with it in the vegetable kingdom in semi-combustion; and charcoal, a substance especially worthy of attention on other accounts, exhibits a black colour. Again, if woods--for example, boards, owing to the action of light, air, and moisture, are deprived in part of their combustibility, there appears first the grey then the black colour. So again, we can convert even portions of animal substance to charcoal by semi-combustion. 499. In the same manner we often find that a sub-oxydation takes place in metals when the black colour is to be produced. Various metals, particularly iron, become black by slight oxydation, by vinegar, by mild acid fermentations; for example, a decoction of rice, &c. 500.