with water, at that temperature (212° F.) the colour is decomposed and
injured, for some of the methyl chloride in the compound is driven off.
In fact, by stronger heating we may drive off all the methyl chloride
and get the original Methyl Violet back again.

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.e._ destroy the basicity of
bases. Again, by oxidising phenol with oxalic acid and vitriol, we get a
colour dyeing silk orange, namely, Aurin, HO.C[C_{6}H_{4}(OH)]_{3}. This
is also an acid or phenolic dye, as a glance at its formula will show
you. Its compound atom bristles, so to say, with phenol-residues, as
some of the aniline dyes do with aniline residue-groups.

We come now to a peculiar but immensely important group of colours known