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.
Generally, however, these appearances of colour are of so mutable a nature that chemists look upon them as deceptive tests, at least in the nicer gradations. For ourselves, as we can only treat