_Dimethylaniline_ is obtained by heating aniline, aniline hydrochloride, and methyl alcohol. _Diethylaniline_ is prepared in an analogous way with the use of ethyl alcohol. By the action of nitrous acid (sodium nitrite and hydrochloric acid) on the acid solution of the last-named compound the _nitroso compounds_ are formed. _Sulphonic acids_ arise by the action of concentrated or fuming sulphuric acid on the corresponding bodies of the aromatic series: benzene disulphonic acid from benzene and fuming sulphuric acid, &c. _Phenols_ and _cresols_ are obtained pure from tar distillation. The remaining hydroxyl derivatives (resorcin, α- and β-naphthol, &c.), are generally obtained by the action of concentrated caustic soda on aromatic sulphonic acids. The most important aromatic aldehyde, _benzaldehyde_, is obtained from toluene; on introducing chlorine at boiling temperature benzyl chloride is first formed, then benzal-chloride and finally benzo-trichloride. In heating benzal-chloride with milk of lime (under pressure) benzaldehyde is formed (C₆H₅COH). _Picric acid_ and _naphthol yellow_ belong to the _nitro dyestuffs_; the last named is obtained by sulphonating α-naphthol with fuming sulphuric acid and by the action of nitric acid on the sulphonated mixture. Nitroso derivatives of aromatic phenols yield (with metal oxides) the material for production of nitroso dyestuffs. To these belong naphthol green, &c. The most important _azo dyestuffs_ technically are produced in principle by the action of nitrous acid on the aromatic amines. The amido compound is converted into the diazo salt by treatment with sodium nitrite in acid solution. Thus diazo-benzene is made from aniline. Diazo compounds are not usually isolated but immediately coupled with other suitable compounds—amido derivatives, phenols—i.e. converted into azo compounds. [Illustration: FIG. 26.—Nitrating Plant (_after Leymann_) I Nitric acid II Balance III Storage tank IV Nitrating pan V Waste acid tank VI Acid egg VII Hydrocarbon VIII Balance IX Storage tank X Washing vessel XI Centrifugal machine XII Egg - - - Exhaust ventilation pipe.] The combination of the two constituents takes place at once and quantitatively. The colour is separated from the aqueous solution by salting-out, and is then put through a filter press. The reactions are carried out generally in wooden vats arranged in stages. Besides a second, a third constituent can be introduced, and in this way naphthol—and naphthylamine sulphonic acids yield a large number of colouring matters. A very large number of azo dyestuffs can thus be produced by the variation of the first component (the primary base) with the second and again with the third component, but it would carry us too far to deal further with their preparation.