by the infusion of gall-nuts, appears black.
XXXVII.
FIRST EXCITATION OF COLOUR.
501.
In the division of physical colours, where semi-transparent mediums were considered, we saw colours antecedently to white and black. In the present case we assume a white and black already produced and fixed; and the question is, how colour can be excited in them?
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