THE AIR AND LIFE. 529 



Living beings are adapted for life in an atmosphere containing one- 

 fourth oxygen and tliree-fonrths nitrogen. Experiments show that if 

 the proi)ortion of oxygen be decreased even by one fourth the air can 

 not sustain lile. The adai)tation of living beings to the atmosphere is 

 thus confiued within narrow bounds, and it is therefore proper to ask 

 whether a variation in the opposite direction, that is, by excess of oxy- 

 gen, would not likewise be injurious to life. Paul Bert contributed 

 much toward a solution of this problem. Experiments have established 

 a fact that seems very strange at first, but which will less surprise those 

 accustomed to consider the adaptation of living beings to their envir- 

 onment. Oxygen, the vital and pre-eminently vivifying gas, is also a 

 virulent ]>oison, not only for animals, but for plants; for cells as well as 

 for the entire organism. If the tension of the oxygen of the air be 

 raised to a certain degree, or, what amounts to the same thing, if its 

 volume be increased to a certain proi:>ortion, that air becomes at 

 once a death-dealing agent. This can be demonstrated in two ways, 

 either by subjecting the animal or plant to an abnormal atmospheric 

 pressure or by placing it in air in which the proportion of oxygen 

 has been artificially increased. In both instances the same phenomena 

 take place and death soon su})ervenes. The cause for this is not well 

 known as regards i)lants, but Paul Bert has shown that animals die in 

 an atmosphere overcharged with oxygen as soon as their blood contains 

 one-third more than the normal ]>roi)ortion of oxygen. The reason for 

 this is that the hemoglobin in the blood, coming in contact with such 

 atmosphere, becomes saturated with oxygen, and after that a por- 

 tion of that gas at once dissolves in the blood, or rather in its liquid 

 serum. Therein lies tlie cause of all the mischief, that oxygen carried 

 in the serum is dissolved, free, uncombined, and on coming in contact 

 with the tissues it kills them. Here we have the method of the proc- 

 ess, but the cause is not yet understood. We can only state the fact 

 that tissues can not stand free oxygen and will take and utilize that 

 gas only by borrowing it from the red globules which convey it in their 

 hemoglobin as already explained. In other words living tissues absorb 

 oxygen indirectly and will not tolerate it when directly suppli<Ml. 



Notwithstanding all this, oxygen is none the less a powerful thera- 

 peutic agent. Like all poisons, it may be administered in beneficial 

 doses and a salutary latitude exists between the normal (luantity found 

 in blood and that at which danger would begin. 



This poisonous property of excessive oxygen is one of the most curi- 

 ous facts that recent years have brought to our notice, and it is so 

 clear, so marked, that it can be no longer oi)en to doubt. 



On the other hand, a general statement that without oxygen there 

 can be no life would be quite incorrect. Pasteur's investigations have 

 shown that while certain microbes can live only with air and oxygen, 

 there are others, which he has called '' anaerobic," that thrive best 

 when deprived of air. It is so with the micro-organisms which cause 

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