One other series of azo-compounds must be briefly referred to. It has long been known that aniline and toluidine when heated with sulphur evolve sulphuretted hydrogen and give rise to thio-bases, that is, aniline or toluidine in which the hydrogen is partly replaced by sulphur. One of the toluidines treated in this way is transformed into a thiotoluidine which, when diazotised and combined with one of the disulpho-acids of beta-naphthol, forms a red azo-dye, introduced by Dahl & Co. in 1885 as "thiorubin." By modifying the conditions of reaction between the sulphur and the base, it was found in 1887 by Arthur Green, that a complicated thio-derivative of toluidine could be produced which possessed very remarkable properties. The sulpho-acid of the thio-base is a yellow dye, which was named by its discoverer "primuline." Not only is primuline a dye, but it contains an amido-group which can be diazotised. If therefore the fabric dyed with primuline is passed through a nitrite bath, a diazo-salt is formed in the fibre, and on immersing the latter in a second bath containing naphthol or other phenol or an amine, an azo-dye is precipitated in the fibre. By this means there are produced valuable "ingrain colours" of various shades of red, orange, purple, &c. So much for the azo-dyes, one of the most prolific fields of industrial enterprise connected with coal-tar technology. From the introduction of aniline yellow in 1863 to the present time, about 150 distinct compounds of this group have been given to the tinctorial industry. Of these over thirty are cotton dyes containing two azo-groups. Sombre shades, rivalling logwood black, bright yellows, orange-reds, browns, violets, and brilliant scarlets equalling cochineal, have been evolved from the refuse of the gas-works. The artificial colouring-matters have in this last case once again threatened a natural product, and with greater success than the indigo synthesis, for the introduction of the azo-scarlets has caused a marked decline in the cochineal culture.