There is one other use for aniline yellow which dates from the year of its discovery, when Dale and Caro found that by adding sodium nitrite to aniline hydrochloride and heating the mixture, a blue colouring-matter is produced. The latter was introduced in 1864 under the name of "induline." It was shown subsequently by the scientific researches of several chemists that the blue produced by Dale and Caro's method results from the action of the aniline salt on the aniline yellow, which is formed by the action of the nitrous acid on the aniline and aniline salt. This explanation was proved to be correct in 1872 by Hofmann and Geyger, who prepared the colouring-matter by heating aniline yellow and aniline salt with alcohol as a solvent. These chemists established the composition and gave it the name of "azodiphenyl blue." Later, viz. in 1883, the manufacture was improved by Otto Witt and E. Thomas, and the dye, under the old name of "induline," is now largely manufactured by first preparing aniline yellow and then heating this with aniline and aniline salt. The colouring-matter as formed by this method is basic and insoluble in water; it is made acid and soluble by treatment with sulphuric acid, which converts it into a sulpho-acid. Induline belongs to the sober-tinted colours, and produces a shade somewhat resembling indigo. Closely related thereto is a bluish-grey called "nigrosine," obtained by heating nitrobenzene with aniline, as well as a certain bluish by-product obtained during the formation of magenta, and known as "violaniline." It will be convenient here to pause and reflect upon the great industrial importance of the two coal-tar hydrocarbons upon which we have thus far concentrated our attention. Their uses are by no means exhausted as yet, but they have already been made to account for such a number of valuable products that the reader may find it useful to have the results presented in a collected form. This is given below as a chronological summary-- 1856. Mauve discovered by Perkin; leading to manufacture of aniline. 1860. Arsenic acid process for magenta discovered; leading to manufacture of arsenic acid. 1860. Aniline blue discovered; leading in 1862 to soluble and Nicholson blues. 1861. Methyl violet discovered; manufactured in 1866; leading to a new use of copper salts as oxidizing agents, and to the manufacture of dimethylaniline. 1862. Hofmann violets discovered; leading to manufacture of methyl iodide from iodine, phosphorus, and wood spirit. 1862. Phosphine (chrysaniline) discovered in crude magenta. 1863. Aniline black introduced; leading to a new use for potassium chlorate and copper salts, and to the manufacture of aniline salt. 1863. Aniline yellow introduced, the first azo-colour. 1864. Induline discovered; leading to new use for aniline yellow.