180 AIR AND LIFE. 



nut exceed certain limits. But beyond these limits danger exists for 

 both animals and man, and while tbe effects are not exactly the same 

 for all species, and do not occur at exactly the same altitude with all 

 species, or even individuals of the same species, the general fact remains 

 tbat at high altitudes, or under very low pressures, life is more or less 

 endangered from different causes. In order to ascertain these causes 

 it is not convenient to take men or animals into high altitudes, as the 

 experimenter would be apt to be also influenced by the diminution of 

 pressure, in consequence of which the value of his'observations might 

 be considerably reduced. A better method, easily available, is that 

 used in laboratories, of providing large or small air-proof chambers in 

 which the pressure may be increased or diminished at will, so tbat, 

 without going out of the laboratory, the same patient or animal may 

 be subjected by turn to the pressure which reigns at the bottom of the 

 deepest mine, or even to far higher pressure, amounting to 800 or 1,000 

 atmospheres, and to that met on the top of the highest peak of the 

 Himalayas, or at twice or three times that height in the lightest of 

 balloons. With such instruments observation becomes easy, and is 

 effected under the most favorable circumstances, as the operator is able 

 to obtain at a few moments' notice exactly the amount of pressure he 

 wishes to have. 



The influence of those extreme pressures, high or low, where life 

 becomes endangered, was very fully investigated by Jourdanet, and 

 afterwards by Paul Bert, and those investigations have taught us by 

 what means they become dangerous. The limits of pressure within 

 which no harm occurs are variable according to species. All terres- 

 trial and aquatic animals may and do resist certain variations in pres- 

 sure, whether above or below the average. Man, for instance, can 

 work at a kilometer below the sea level without any injury, and he can 

 travel to the height of 5 or 6 kilometers in the atmosphere without being 

 necessarily affected by the decrease of pressure. It is the same with 

 birds and mammals, and surface or shore fishes may go pretty deep in 

 the seas without experiencing any unpleasant effects, while deep-sea 

 fish may travel upward for some time before reaching the danger line, 

 so to speak. But for all organisms there are limits in tbe variation of 

 pressure which can not be transgressed with impunity; there are limits 

 beyond which life is destroyed. 



How is death induced in such cases? We must consider the two 

 different cases in turn, and shall begin with the effects of diminished 

 pressure. 



Four hundred years have now elapsed since a Jesuit missionary, 

 Acosta, left us an excellent description of the accidents which attend 

 ascensions in high mountains, or important diminution of pressure. 

 "While ascending a mountain in Peru," writes Acosta, "I was sud- 

 denly affected by so strange and so mortal an evil that I nearly dropped 

 from iny horse to the ground. * * * I was alone with an Indian, and 



