Marked injury to health and distress to workers through acid fumes are sometimes caused by the denitration of the waste mixture of sulphuric and nitric acids in the nitrating process, that is, by the separation of nitric acid from the acid mixture. This denitration takes place usually in the Glover towers of the lead chamber system which is often associated with the manufacture of aniline. The mixed nitro-compounds of the waste acids, however, are often not completely condensed, but pass through the chambers and Gay-Lussac towers and escape into the air, whence arises the constant smell of nitrobenzene in aniline factories (Leymann). In the production of naphthylamine and recovery of chlorinated products, escaping chlorine should be led into chloride of lime chambers, hydrochloric acid fumes into towers to be absorbed by water and milk of lime or a solution of soda.

In aniline factories danger can scarcely be wholly avoided, as the workers, on the one hand, come into contact with poisonous substances, nitrobenzene, aniline, &c., and on the other hand, in spite of all technical hygienic measures, can hardly help breathing in some of the aniline. Apart from the technical regulations, therefore, there must be insistence on cleanliness of the workrooms, personal cleanliness on the part of the workers (washing, baths, working suits, cloak-rooms, &c.). Besides this, contact with aniline, nitrobenzene, &c., wetting of the body and clothes with these substances, and, especially spilling, splashing, and scattering these fluids must be carefully avoided. The workers require to be suitably instructed as to the symptoms of nitrobenzene and aniline poisoning, and the right steps to take, if poisoned. The oxygen apparatus must always be at hand, ready for use; the workers must be instructed how to use it. Further, workers, especially those newly employed, must be under supervision in order that assistance may be rendered them on the first signs of poisoning; medical assistance ought to be within easy reach. Workers also should know of the tendency of aniline to cause cancer of the bladder.

Precautions against the poisonous nitro-derivatives of benzene (nitrophenol, picric acid, &c.), which are in the form of poisonous dust, must take the form of entirely closed-in grinding and packing apparatus, or, at all events, removal of the dust at its source.

Among official regulations may be mentioned the Prussian Ministerial Edict, dated December 18, 1908, as to purification and storage of benzene, and further the Regulations dated December 13, 1907, and December 30, 1908, in force in Great Britain for the manufacture of nitro- and amido-derivatives of benzene, and the manufacture of explosives with use of dinitrobenzene or dinitrotoluene.

VI
PREVENTIVE REGULATIONS—THE EXTRACTION OF METALS (SMELTING WORK IN GENERAL)

Danger is incurred when the furnace leaks, a condition which generally occurs in the course of time, or if gases escape during the necessary manipulations through the working doors. This can be avoided by maintaining the walls in as air-tight a state as possible; but as very small leakages are almost unavoidable the best course is to so regulate the draught in the furnace (by means of fans) that a slight negative pressure always exists in it. Naturally, poisonous gases escaping from the furnace such as sulphur dioxide, carbonic oxide, carbon dioxide, and hydrocarbons require to be drawn away and rendered harmless. This can often be done by merely conducting them into the main flue. Gases containing carbonic oxide possess high heating capacity, and their escape can usually be prevented by suitable cupola bells. They can be led away in impervious conduits and utilised for heating purposes or for driving gas engines. Entering the flues for cleaning or repairing purposes is especially dangerous; and as it is difficult to isolate one portion entirely from another, such operations might well be carried on by persons equipped with breathing apparatus (smoke helmets or oxygen apparatus).

In roasting operations handwork can be largely replaced by furnaces worked mechanically. If the gases generated are rich in sulphur dioxide they can be utilised for the manufacture of sulphuric acid or for the production of liquid sulphur dioxide either directly or after concentration; if not, they must be rendered harmless by treatment with milk of lime in absorption towers. Other methods of rendering the sulphur dioxide (unsuited for manufacture of sulphuric acid) harmless depend on treatment with minerals containing calcium carbonate, or magnesium or aluminium hydrate, sodium sulphide, &c. Sometimes the sulphurous gases are led into blast furnaces containing oxide of iron and coal (so as to form sulphide of iron) or are absorbed by means of moist scraps of sheet iron or brown coal or peat briquettes.

Use of chlorine compounds in the extraction of metals from ores (silver, copper) causes evolution of chlorine and hydrochloric acid vapour. These should be dealt with in absorption towers. Metallic fumes are collected by suitable condensing arrangements. Flue dust is retained in flue dust chambers, but in the cleaning of such condensing flues and chambers danger to the workers is considerable and they should be equipped with respirators, working suits, &c. Personal hygiene must be insisted on.

Iron

(See also pp. [146-51])