While silver approximates most to pure white,--nay, really represents pure white, heightened by metallic splendour,--steel, tin, lead, and so forth, incline towards pale blue-grey; gold, on the other hand, deepens to pure yellow, copper approaches a red hue, which, under certain circumstances, increases almost to bright red, but which again returns to a yellow golden colour when combined with zinc. 510. But if metals in their pure state have so specific a determination towards this or that exhibition of colour, they are, through the effect of oxydation, in some degree reduced to a common character; for the elementary colours now come forth in their purity, and although this or that metal appears to have a particular tendency to this or that colour, we find some that can go through the whole circle of hues, others, that are capable of exhibiting more than one colour; tin, however, is distinguished by its comparative inaptitude to become coloured. We propose to give a table hereafter, showing how far the different metals can be more or less made to exhibit the different colours. 511. When the clean, smooth surface of a pure metal, on being heated, becomes overspread with a mantling colour, which passes through a series of appearances as the heat increases, this, we are persuaded, indicates the aptitude of the metal to pass through the whole range of colours. We find this phenomenon most beautifully exhibited in polished steel; but silver, copper, brass, lead, and tin, easily present similar appearances. A superficial oxydation is probably here taking place, as may be inferred from the effects of the operation when continued, especially in the more easily oxydizable metals. 512. The same conclusion may be drawn from the fact that iron is more easily oxydizable by acid liquids when it is red hot, for in this case the two effects concur with each other. We observe, again, that steel, accordingly as it is hardened in different stages of its colorification, may exhibit a difference of elasticity: this is quite natural, for the various appearances of colour indicate various degrees of heat.[1] 513. If we look beyond this superficial mantling, this pellicle of colour, we observe that as metals are oxydized throughout their masses, white or black appears with the first degree of heat, as may be seen in white lead, iron, and quicksilver. 514. If we examine further, and look for the actual exhibition of colour, we find it most frequently on the _plus_ side. The mantling, so often mentioned, of smooth metallic surfaces begins with yellow. Iron passes presently into yellow ochre, lead from white lead to massicot, quicksilver from æthiops to yellow turbith. The solutions of gold and platinum in acids are yellow. 515. The exhibitions on the _minus_ side are less frequent. Copper slightly oxydized appears blue. In the preparation of Prussian-blue, alkalis are employed. 516.