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
as the azo-dyes, and these can be basic or acid, or of mixed kind. Just
suppose two ammonia groups, NH_{3} and NH_{3}. If we rob those nitrogen
atoms of their hydrogen atoms, we should leave two unsatisfied nitrogen
atoms, atoms with an exceedingly keen appetite represented in terms of
hydrogen atoms as N*** and N***. We might suppose a group, though of two N
atoms partially satisfied by partial union with each other, thus--N:N--.
Now this group forms the nucleus of the azo-colours, and if we satisfy a
nitrogen at one side with an aniline, and at the other with a phenol, or
at both ends with anilines, and so on, we get azo-dyes produced.
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