We have now to turn to another chapter in the history of dimethylaniline. In 1876, Lauth discovered a new colour test for one of the diamidobenzenes. By heating this base with sulphur, and oxidizing the product, a violet colouring-matter was formed, and the same compound was produced by oxidizing the base in an aqueous solution in the presence of sulphuretted hydrogen. Lauth's violet was never manufactured in quantity because the yield is small; but in the hands of Dr. Caro the work of Lauth bore fruit in another direction. Instead of using the diamidobenzene, Caro used its dimethyl-derivative, and by this means obtained a splendid blue dye, which was introduced under the name of "methylene blue." Here again we find scientific research reacting on technology. A few words of chemical explanation will make this manufacture intelligible. By the action of reducing agents on nitro and nitroso-compounds, the nitro and nitroso-group become converted into the amido-group. Thus when nitrobenzene is reduced by iron and an acid we get aniline; similarly when nitrosodimethylaniline is reduced by zinc and an acid we get amidodimethylaniline, and this is the base used in the preparation of methylene blue. By oxidizing this base in the presence of sulphuretted hydrogen, the colouring-matter is formed. Other methods of arriving at the same result were discovered and patented in due course, but the various processes cannot be discussed here. Lauth's violet and methylene blue became the subjects of scientific investigation in 1879 by Koch, and in 1883 a series of brilliant researches were commenced by Bernthsen which extended over several years, and which established the constitution of these compounds. It was shown that they are derivatives of diphenylamine containing sulphur as an essential constituent. The parent-compound is diphenylamine in which sulphur replaces hydrogen, and is therefore known as thiodiphenylamine. It can be prepared by heating diphenylamine with sulphur, and is sometimes called thiazine, because it is somewhat analogous in type to azine. We must therefore credit dimethylaniline with being the industrial generator of the thiazines. The blue is largely used for cotton dyeing, producing on this fibre when properly mordanted an indigo shade. By the action of nitrous acid the blue is converted into a green known as "methylene green."