The azine dyes are basic, and mostly of a red or pink shade; they are somewhat fugitive when exposed to light, but possess a certain value on account of their affinity for cotton, and the readiness with which they can be used in admixture with other colouring-matters. Some of the best known are made by oxidizing certain derivatives of aniline or toluidine, in the presence of these or analogous bases. To make this intelligible a little more chemistry is necessary. Aniline is a derivative of benzene in which one atom of hydrogen is replaced by the residue of ammonia. Ammonia is composed of one atom of nitrogen and three atoms of hydrogen; benzene is composed of six atoms of carbon and six of hydrogen. If one atom of hydrogen is supposed to be withdrawn from ammonia, there remains a residue called the amido-group, and if we imagine this group to be substituted for one of the hydrogen atoms in benzene, we have an amido-derivative, _i.e._ amidobenzene or aniline. Similarly, the toluidines are amidotoluenes. If two hydrogen atoms in benzene or toluene are replaced by two amido-groups, we have diamidobenzenes and diamidotoluenes, which are strongly basic substances, capable of existing in several isomeric modifications. Certain of these diamido-compounds when oxidized in the presence of a further quantity of aniline, toluidine, and such amido-compounds, give rise to unstable blue products, which readily become transformed into red dyes of the azine group. Some azine dyes are produced by another method, which is instructive because it brings us into contact with a derivative of dimethylaniline which figures largely in the coal-tar colour industry. By the action of nitrous acid on this base, there is produced a compound known as nitrosodimethylaniline, which was discovered by Baeyer and Caro in 1874, and which contains the residue of nitrous acid in place of one atom of hydrogen. The residue of nitric acid which replaces hydrogen in benzene is the nitro-group, and the compound is nitrobenzene. The analogy with nitrous acid will therefore be sufficiently understood--the residue of this acid is the nitroso-group, and compounds containing this group are nitroso-derivatives. In 1879, Otto Witt found that the nitroso-group in nitrosodimethylaniline acted as an oxidizing group, and enabled this compound to act upon certain diamido-derivatives of benzene and toluene, with the formation of unstable blue compounds, which on heating the solution changed into red colouring-matters of the azine group. This process soon bore fruit industrially, and azines of a red, violet, and blue shade were introduced under the names of neutral red, violet, and blue, Basle blue, &c., some of these surviving at the present time.