derivatives.  Methyl Violet is a colour we have already referred to, and its chemical structure is still more complex, but it also is built up of aniline materials, and so is a basic aniline colour.  Now it is possible for the colour-maker to prepare a very fine green dye from this beautiful violet (Methyl Violet).  In fact he may convert the violet into the green colour by heating the first under pressure with a gas called methyl chloride (CH_{3}Cl).  Methyl Violet is constructed of aniline or substituted aniline groups; the addition of CH_{3}Cl, then, gives us the Methyl Green.  But one of the misfortunes of Methyl Green is that if the fabric dyed with it be boiled with water, at that temperature (212 deg.  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 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.  The number of coal-tar colours is thus very great, and the variety also.