Certain investigations that were being undertaken in this laboratory required the development of an apparatus by which vapors could be applied to the skin in a uniform and reproducible manner. None of those previously employed for this purpose or described by others appeared to be satisfactory. Thus Machle and Zwick1 were interested only in qualitative reactions to gasoline fractions. Their procedure was to fill a series of test tubes about half full with different fractions of the gasoline. These tubes were lined up in a rack, and the forearm of the patient was laid on the open mouths of the test tubes. Miescher2 used small glass cylinders the bottoms of which fitted into openings in a sheet of celluloid. The top of each tube was closed with a stopper, and just below the stopper was placed a cotton wad which was moistened with drops of the irritating