RESPIRATION 361 



effect, I may mention a recent personal experience. Dr. Priestley 

 and I had gone to a barometric pressure of about 360 mm. in a 

 steel chamber to test a piece of apparatus ; and, being anxious to 

 test our Eustachian tubes, we opened the inlet tap full, so as to 

 raise the pressure to nearly normal within about a minute, as in a 

 iiose dive of about 18,000 feet. Our ears were all right, but I was 

 alarmed to see the filament of the electric lamp suddenly become 

 intensely bright, as if it were about to fuse ; and on hastily pushing 

 the door open at the end of the decompression I inquired what had 

 gone wrong with the voltage. The appearance was of course only 

 subjective. I had forgotten the increase of oxygen pressure, and 

 had only been thinking of the mechanical effect on the eardrums. 



Nothing in subsequent investigation has shaken Paul Bert's 

 conclusions as to the effects of gases being dependent on their 

 partial pressures, though the scientific world has taken a long 

 time to assimilate his reasoning, s© that much of what has been 

 subsequently written on the subject of high and low atmospheric 

 pressures has been simply out of date. On a number of points, 

 however, later investigations have thrown new light. To take one 

 quite minor point first, the action of CO in air does not depend 

 upon its partial pressure, ^ince the higher the pressure of an 

 atmosphere containing CO is raised the more innocuous does the 

 CO become, from the causes already discussed in Chapters IV 

 and VI L But at a constant partial pressure of oxygen the physio- 

 logical action of CO depends upon its partial pressure. There may 

 be other apparent exceptions to Paul Bert's rule, but we may be 

 confident that they will also turn out to be only apparent. 



In his experiments Paul Bert took into direct account only the 

 pressure of oxygen and other gases in the inspired air. But we 

 have already seen that what directly matters is the gas pressures 

 in the alveolar air. When the barometric pressure is lowered the 

 alveolar oxygen pressure falls at a greater proportional rate than 

 the oxygen pressure of the inspired air. This is because, even 

 though the breathing is increased, which would in itself tend to 

 keep up the alveolar oxygen pressure, and may nearly prevent 

 the alveolar CO2 percentage from rising, the percentage of 

 aqueous vapor is constantly rising. At a barometric pressure of 

 47 mm. no air at all would enter the lungs, since the pressure of 

 aqueous vapor would be 47 mm., and the liquids of the body would 

 from this cause alone be just about their boiling point; as a matter 

 of fact they would boil at a higher pressure, as they contain much 

 free CO2. At a pressure of 100 mm. in an atmosphere of pure 



