THE investigation wh'ich forms the subject of the present paper arose out of a series of experiments on the attenuation of microbes by oxygen at high pressure. Part of this series was carried out with the view of ascertaining the effect of the oxygen on animals which had been infected. It soon became apparent, however, that the oxygen at a tension of over 100 0/o of an atmosphere produced pneumonia in the normal animal. It was therefore necessary to carry out a preliminary research in regard to this.The literature of respiration records in connection with this question a considerable number of investigations, the most importaint of which were carried out by Lavoisier aud others immediately after the promulgation of the combustion theory of respiration. According to various authors, definite effects were obtained by breathing pure oxygen. The respiratory exchange was increased, the circulation quickened, congestion of the lungs, or even inflammation and death, occurred. The theory was that addition of oxygen increased the pulmonary combustion, and thereby produced these pathological changes. This result was controverted by Regnault and Reiset in their classical investigation'. They showed that no increase in oxidation occurred and no pathological changes ensued on the exposure of animals to atmospheres rich in oxygen. The question of the effect of oxygen took a new form in Paul Bert's research on the effects on animals of variations of baromletric pressure.He discovered the fundamental law that the effects on all living organisms arising from variations in barometric pressure are entirely the result of the tensions at which the oxygen is nmaintained in the

Average 35X2 0/0 30X2 0/0 In this experiment we intended to ascertain whether a reduction by '023 0/0 of the percentage of 0o breathed would be followed by a corresponding reduction in the saturation of the blood. No appreciable reduction was, however, observed within 11 hours (see p. 514).

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