BENZENE AND ITS HOMOLOGUES _Benzene_ (C₆H₆) is a characteristically smelling (aromatic) liquid which boils at 80·5° C. Acute benzene poisoning, which plays an important part as an industrial poisoning, is caused by inhalation of benzene fumes. The various kinds of benzol used commercially contain, besides benzene, alkyl benzenes, especially _toluene_ (methylbenzene, C₆H₅.CH₃, boiling-point 111° C.); _xylene_ (dimethylbenzene, C₆H₄[CH₃]₂, boiling-point 140° C.); _pseudocumene_ and _mesitylene_ (tri-methylbenzene, C₆H₃[CH₃]₃, boiling-point 169° or 163° C.); the regular presence of _thiophene_ (C₄H₄S, boiling-point 84° C.) in commercial benzol must also be taken into account. Industrial benzol poisoning arises, therefore, as a rule, not from the action of pure benzene vapour, but from fumes which contain a mixture of the compounds mentioned. The course run by industrial benzol poisoning is often very acute, if large quantities are inhaled—death occurring suddenly, after a short illness with symptoms of vertigo. Gradual inhalation of lesser quantities gives rise to headache, giddiness, malaise, then twitchings appear which develop into convulsions, and lastly unconsciousness. In order to ascertain in what manner the various substances contained in commercial benzol share in the poisonous effect, experimental research seemed to me to be indispensable, especially as published statements so far gave no accurate data. Two cases of industrial benzol poisoning have given rise to close experimental research upon the poisonous nature of benzene. Lewin undertook experiments on animals; which he confined under bells and caused to inhale fumes of chemically pure and impure benzene. He mentions that even at comparatively low concentration poisoning results, and indeed more readily and certainly from the action of impure than pure benzene. Lewin found that when air was made to flow slowly first through benzene and then into the bell, symptoms of paralysis, convulsions, and unconsciousness showed themselves in from four to six minutes. After-effects by this means could not be observed. Lewin maintains, however, that in man even slight acute action of benzene can be followed by after-effects (giddiness, sickness, headache, distress in breathing, and oppression of the heart).