Some of these bases and their homologues are found in the evil-smelling oil produced by the destructive distillation of bones (Dippel's oil, or bone oil), and the group is frequently spoken of as the pyridine group. The colouring-matter described as phosphine, obtained as a by-product in the manufacture of magenta (p. 94), is a derivative of acridine, and a yellow colouring-matter discovered by Rudolph in 1881, and obtained by heating the acetyl derivative of aniline with zinc chloride, is a derivative of a homologue of quinoline. This dye-stuff, known as "flavaniline," is no longer made; but it is interesting as having led to the discovery of the constitution of phosphine by O. Fischer and Koerner in 1884. The antipyretic medicines which we have first to consider are derivatives of quinoline. This base was discovered in coal-tar by Runge in 1834, and was obtained by Gerhardt in 1842 by distilling cinchonine, one of the cinchona alkaloids, with alkali. Now it is of interest to note that the quinoline of coal-tar is of no more use to the technologist than the aniline; these bases are not contained in the tar in sufficient quantity to enable them to be separated and purified with economical advantage. If the colour industry had to depend upon this source of aniline only, its development would have been impossible. But as chemistry enabled the manufacturer to obtain aniline in quantity from benzene, so science has placed quinoline at his disposal. This important discovery was made in 1880 by the Dutch chemist Skraup, who found that by heating aniline with sulphuric acid and glycerin in the presence of nitrobenzene, quinoline is produced. The nitrobenzene acts only as an oxidizing agent; the amido-group of the aniline is converted into a group containing carbon, hydrogen, and nitrogen, _i.e._ the pyridine group. The discovery of Skraup's method formed the starting-point of a series of syntheses, which resulted in the formation of many products of technical value. In all these syntheses the fundamental change is the same, viz. the conversion of an amidic into a pyridine group. We may speak of the amido-group as being "pyridised" in such processes. Thus alizarin blue, which is formed by heating nitro-alizarin with glycerin and sulphuric acid, results from the pyridisation of the nitro-group. By an analogous method Doebner and v. Miller prepared a homologue of quinoline (quinaldine) in 1881, by the action of sulphuric acid and a certain modification of aldehyde known as paraldehyde on aniline. Quinoline and its homologue quinaldine have been utilized as sources of colouring-matters. A green dye-stuff, known as quinoline green, was formerly made by the same method as that employed for producing the phosgene colours by Caro and Kern's process (p. 106). The phthalein of quinaldine was introduced by E. Jacobsen in 1882 under the name of quinoline yellow, a colouring-matter which forms a soluble sulpho-acid by the action of sulphuric acid.