RESPIRATION 355 



ing state regulations might need modification to enable the im- 

 provement to be introduced. In any case there is now no justifica- 

 tion for imperiling men's lives by methods of decompression which 

 are known to give imperfect protection. 



At present the tendency of the supervising medical officers is 

 to shorten the periods of work at the face under high pressure; 

 and of course the period of decompression may then be shortened 

 also. While this may cover the physiological aspects of the prob- 

 lem, it is evidently very uneconomical as compared with the 

 method above suggested. 



Not only may increased partial pressures of nitrogen and CO 2 

 cause trouble, but also increased pressure of oxygen. The poison- 

 ous action of oxygen at high partial pressure was discovered by 

 Paul Bert; and his numerous and very thorough experiments on 

 the subject are described in his famous book. There is a popular 

 belief, based on the supposed similarity between life and com- 

 bustion, that the breathing of oxygen at a high partial pressure 

 must quicken the processes of life, and Paul Bert's experiments 

 on the effects of a high partial pressure seem to have been begun 

 with the view of testing this belief. He found that when the partial 

 pressure of oxygen exceeds three or four atmospheres, very re- 

 markable tonic convulsions are produced in warm-blooded ani- 

 mals, and they soon die. More remarkable still, perhaps, their 

 body temperature falls in the compressed oxygen, and the con- 

 sumption of oxygen and production of CO 2 are markedly di- 

 minished. The oxygen acts as a poison. 



He then extended his observations to other forms of life besides 

 warm-blooded animals, and proved conclusively that for life in 

 every form, including the very lowest, oxygen at high pressure 

 is a poison. Plants, infusoria, and bacteria are killed just as 

 certainly as the higher animals. His experiments left no doubt that 

 it is the partial pressure of oxygen, and not mere mechanical 

 pressure, that matters. When air was used instead of pure oxygen, 

 the pressure required to produce fatal effects was nearly five times 

 as great as when pure oxygen was used, but the pressure of oxy- 

 gen was the same. He also found that oxygen pressures of less 

 than one atmosphere would kill or retard the growth of various 

 small organisms of different classes in the animal kingdom, and of 

 plants ; and he came to the conclusion that any increase over the 

 normal oxygen pressure of ordinary air is more or less detrimental 

 to living organisms directly exposed to it. He had discovered a 

 biological fact of the most far-reaching significance. 



