One other development of this branch of manufacture has yet to be recorded. The new departure was made in 1883 by Caro and Kern, who patented a process for the synthesis of colouring-matters of this group. In this synthesis a gas called phosgene is used, the said gas having been discovered by John Davy in 1811, who gave it its name because it is formed by the direct union of chlorine and carbon monoxide under the influence of sunlight. Caro and Kern's process is the first technical application of Davy's compound. By the action of phosgene on dimethylaniline and analogous bases in the presence of certain compounds which promote the chemical interaction, a number of basic colouring-matters of brilliant shades of violet ("crystal violet") and blue ("Victoria blue," "night blue") are produced, these being all members of the triphenylmethane group. One of these dyes is a fine basic yellow known as "auramine," which is a derivative of diphenylmethane. TAR. (Light oil) | | | | ---------------- -------- | | / Benzyl chloride Benzene Toluene { Benzal chloride } | | | \ Benzotrichloride}<- | | ------------ | Aniline-> Magenta (and Phosphine)<-Toluidine | | / \ | \ | | / \ | \ ---Benzoic acid and<- Dimethylaniline Diphenylamine | \ | Benzoic aldehyde | | | \ | / | | | Aniline Blues: / | | Hofmann Violet Nicholson and / | | and Iodine Green Soluble / | Diph. Blue / | / ------------------------------------------------ / | | | / Crystal Violet, Methyl Violet | / Auramine, & and Green Malachite other Phosgene dyes Green series From benzene and toluene alone about forty distinct colouring-matters of the rosaniline group are sent into commerce. The relationship of those compounds to each other and to their generating substances is not easy to grasp by those to whom the facts are presented for the first time. The scheme on page 107 shows these relationships at a glance.