But we have coal-tar colours which are not basic, but rather of the
nature of acid,--a better term would be _phenolic_, or of the nature of
phenol or carbolic acid. Let us see what phenol or carbolic acid is. We
saw that water may be formulated HOH, and that benzene is C_{6}H_{6}.
Well, carbolic acid or phenol is a derivative of water, or a derivative
of benzene, just as you like, and it is formulated C_{6}H_{5}OH. You can
easily prove this by dropping carbolic acid or phenol down a red-hot
tube filled with iron-borings. The oxygen is taken up by the iron to
give oxide of iron, and benzene is obtained, thus: C_{6}H_{5}OH gives O
and C_{6}H_{6}. But there is another hydrocarbon called naphthalene,
C_{10}H_{8}, and this forms not one, but two phenols. As the name of the
hydrocarbon is naphthalene, however, we call these compounds naphthols,
and one is distinguished as alpha- the other as beta-naphthol, both of
them having the formula C_{10}H_{7}OH. But now with respect to the
colours. If we treat phenol with nitric acid under proper conditions, we
get a yellow dye called picric acid, which is trinitro-phenol
C_{6}H_{2}(NO_{2})_{3}OH; you see this is no aniline dye; it is not a
basic colour, for it would saturate, _i.
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