Observations in a chemical factory since 1892 showed that of thirty thus affected in the course of ten years twenty-two came into contact with paraffin. Artificial Organic Dye Stuffs (Coal-tar Colours) MANUFACTURE.—The starting-points for the preparation of artificial coal-tar dyes are mainly those aromatic compounds (hydrocarbons) described in the preceding section. Besides these, however, there are the derivatives of the fatty series such as methyl alcohol (wood spirit), ethyl alcohol, phosgene, and, latterly, formaldehyde. The _hydrocarbons of the benzene series_ from tar distillation are delivered almost pure to the colour factory. Of these benzene, toluene, xylene, naphthalene, anthracene, and the phenols, cresols, &c., have to be considered. Further treatment is as follows: 1. Nitration, i.e. introduction of a nitro-group by means of nitric acid. 2. Reduction of the nitrated products to amines. 3. Sulphonation, i.e. conversion to sulphonic acids by means of concentrated sulphuric acid. 4. The sulphonic acids are converted into phenols by fusing with caustic soda. 5. Introduction of chlorine and bromine. _Nitro-derivatives_ are technically obtained by the action of a mixture of nitric and concentrated sulphuric acids on the aromatic body in question. The most important example is _nitrobenzene_. Benzene is treated for several hours in cylindrical cast-iron pans with nitric and concentrated sulphuric acids. The vessel is cooled externally and well agitated. A temperature of 25° C. should not be exceeded. [Illustration: FIG. 25.—Preparation of Intermediate Products in the Aniline Colour Industry (Closed Apparatus), showing Arrangement for Condensation (_after Leymann_)] On standing the fluid separates into two layers: the lower consists of dilute sulphuric acid in which there is still some nitric acid, and the upper of nitrobenzene. The latter is freed of remains of acid by washing and of water by distillation. _Toluene_ and _xylene_ are nitrated in the same way. _Dinitro products_ (such as metadinitrobenzene) are obtained by further action of the nitro-sulphuric acid mixture on the mononitro-compound at higher temperature. For conversion of phenol into _picric acid_ (trinitrophenol) the use of a nitro-sulphuric acid mixture is necessary. The _aromatic bases_ (aniline, toluidine, xylidine) are obtained by reduction of the corresponding nitro-compound by means of iron filings and acid (hydrochloric, sulphuric, or acetic). Thus in the case of _aniline_ pure nitrobenzene is decomposed in an iron cylindrical apparatus, provided with agitators and a condenser, and avoidance of a too violent reaction, by means of fine iron filings and about 5 per cent. hydrochloric acid. After completion of the reaction the contents are rendered alkaline by addition of lime and the aniline distilled over. Manufacture of _toluidine_ and _xylidine_ is analogous.