THE AIR AND LIFE. 539 



pressure of water, wliicli is uiucli denser than air, must be oft'set by a 

 considerable compressiou of air, equal to 3 or 4 atmospheres. When 

 that compression is moderate the disorders are slight, and are confined 

 to a buzzing" sensation in the ears, bleeding of the nose, and benumbing 

 of the limbs; but breathing becomes slower and the pulse more lag- 

 gard. In some cases the nervous system exhibits abnormal excite- 

 ment, similar to that produced by intoxication. It is quite natural to 

 infer that these accidents are the result of an increase in the tension 

 of carbonic acid and indeed that gas does produce the mischief so 

 long as the compression does not go beyond certain limits, for it accu- 

 mulates in the organism, and, as a natural consequence, suffocates and 

 poisons it. But if heavy pressures are brought iuto play, as was done 

 by Paul Bert, a very uncommon and quite different result is reached. 

 That lamented physiologist, laboring under the Impression that he 

 would retard the fatal effect of compression, charged the air with a 

 large proportion of oxygen for the purpose of combatting the poisonous 

 action of carbonic acid, and was not a little surprised to find that his 

 attempts only resulted in precipitating the catastro])he. By analyzing 

 the phenomena he found that under considerable pressure, over 6 

 atuiospheres, the oxygen of air, by acquiring a very strong tension, 

 becomes a poison in the same manner as it does for an auinml breath- 

 ing under normal pressure in a medium abounding in that gas. And 

 the evidence of the injurious effects of oxygen is furuished by the fact 

 that an animal can easily stand a pressure of 20 atmospheres if the air 

 IS poor ill oxygen, if that gas, being nnne rare, attains a tension but 

 little higher than that which it possesses in the normal air under ordi- 

 nary pressure. When under excessive tension, or in too great abun- 

 dance, Avhich amounts to the saiue thing, oxygen always proves to be a 

 poison of the most dangerous kind, aud it is for this reasou that ani- 

 mals and men die in a normal atmospheric medium so soon as the pres- 

 sure increases beyond certain limits. Whether the compression be 

 swift or slow, it is the excess of oxygen that is fatal. If we leave out 

 of consideration all cases in which the changes of pressure are rapid, 

 and in which, as when the pressure is suddenly lessened, a purely 

 mechanical agent intervenes, we shall generally find that the action of 

 gradual variations is not of a physical nature, but exclusively a chem- 

 ical one, induced by putting the organism in contact with air that is 

 either too rich or too poor in oxygen. 



It is proper to add that habit, here as elsewhere, plays an important 

 part.* For instance, Indians and animals in the Cordilleras are proof 

 against the mountain sickness that attacks travellers, and animals living 

 in great depths undergo pressures that no being on land or in shallow 



* Recent investigations conducted by Messrs. Miintz .and Regnard have proved 

 that when carried to elevated regions or subjected for some time to experimental 

 rarefaction, a person acquires the power of absorbing larger quantities of oxygen. 

 This exTilains the beneficial etfectof a sojourn in the mountains. 



