iron, become black by slight oxydation, by vinegar, by mild acid fermentations; for example, a decoction of rice, &c.
500.
Again, it may be inferred that a de-oxydation may produce black. This occurs in the preparation of ink, which becomes yellow by the solution of iron in strong sulphuric acid, but when partly de-oxydised 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?