AIR AND LIFE. 179 



greatly, it is also a necessity for life, and in the proportions in which it 

 exists in the atmosphere it is just as much a necessity as it would 

 become a fatal danger if it were to be present in larger quantity. 



Such are the relations between air considered from the chemical 

 standpoint arid life as it exists on earth; between air in its normal, 

 un vitiated, average constitution and life as it manifests itself under the 

 present circumstances. 



IV. — Biological Influence of the Atmosphere Considered 

 from the Physical Point of View. 



We must now discuss another side of this complex question, we must 

 deal with air considered as a physical substance, and especially as a 

 substance having weight which presses upon all living organisms. This 

 point of view is not less important than the preceding, and deserves 

 some attention, by reason of the relations which exist between life and 

 atmospheric pressure. 



The atmosphere, as previously noticed, being a physical substance, 

 possesses weight, and exerts a pressure upon the earth and all beings 

 that inhabit it. 1 



As long as men or animals keep near sea level, or do not climb to 

 exceedingly high altitudes, the normal average variations of pressure, 

 as indicated by the barometer, are of small influence, and the much 

 more considerable variations which are encountered when one ascends 

 mountains or goes up in a balloon are not harmful as long as they do 



'The average pressure of the atmosphere varies, as before stated, according to the 

 altitude of the locality, and also in the same locality at different times. At the sea 

 level this average pressure amounts to a little over a kilogram per square centi- 

 meter, hence the total weight supported by an average man is about 18,000 kilograms. 

 At Mexico the average weight per square centimeter goes down to 793 grams; at 

 Quito, to 752; at Antisana, to 639; and it is no difficult matter to obtain the figure 

 which represents the weight supported by man in such localities, when one knows 

 that the skin surface of an average adult is somewhere between 1,400 and 1,500 

 square centimeters. The physicist Haiiy, explaining and commenting upon the cal- 

 culations by means of which the average pressure exerted upon the body is ascer- 

 tained, remarks : "Andthafis the weight which those philosophers of old had to 

 bear and resist who denied weight to the atmosphere." 



This weight or pressure is considerable, but we do not feel it, as all the interior 

 parts of our body exert the same pressure and therefore resist successfully that from 

 the outside. It does not crush us any more than it crushes the soap bubbles, how- 

 ever thin they may be, because in both cases the resistance of internal air or tissues 

 exactly counteracts that of external air. There are very few places in the body 

 where the pressure from within outward does not exactly counteract the opposite 

 pressure, in order to leave all movements perfectly free. Two exceptions, however, 

 must be referred to — that of the pleura, between which no counter pressure exists, so 

 that they are compelled by atmospheric pressure to keep strictly in contact, and that 

 of certain articulations, where the head of a bone so exactly fits into a correspond- 

 ing cavity that there is place for no air between, with the result that the atmospheric 

 pressure forces the former into the latter and keeps it there with sufficient force to 

 resist the counteracting weight of the limb. 



