502.
Here, too, we can say, white that becomes darkened or dimmed inclines to yellow; black, as it becomes lighter, inclines to blue.—Note U.
503.
Yellow appears on the active (plus) side, immediately in the light, the bright, the white. All white surfaces easily assume a yellow tinge; paper, linen, wool, silk, wax: transparent fluids again, which have a tendency to combustion, easily become yellow; in other words they easily pass into a very slight state of semi-transparence.
504.
So again the excitement on the passive side, the tendency to obscure, dark, black, is immediately accompanied with blue, or rather with a reddish-blue. Iron dissolved in sulphuric acid, and much diluted with water, if held to the light in a glass, exhibits a beautiful violet colour as soon as a few drops only of the infusion of gall-nuts are added. This colour presents the peculiar hues of the dark topaz, the orphninon of a burnt-red, as the ancients expressed it.
505.
Whether any colour can be excited in the pure earths by the chemical operations of nature and art, without the admixture of metallic oxydes, is an important question, generally, indeed, answered in the negative. It is perhaps connected with the question—to what extent changes may be produced in the earths through oxydation?
506.