During the discussion following the late Sir Henry Japp's paper on ‘Prevention of compressed air illness’, read at a meeting of the Institute of Structural Engineers on 28 March 1935, I suggested an extension of Haldane's system of decompression which might prove of considerable practical value in tunnel and caisson work (Japp, 1935). Briefly the proposed extension consisted in making use of Dalton's theory of the partial pressure of gases in such a manner that denitrogenization of the blood might be brought about while the workers were still under full pressure, namely, by making them breathe before entering the decompressing chamber an artificial atmosphere containing no nitrogen.