THE 



PHILOSOPHY 



OF 



NATURAL HISTORY 



CHAPTER I. 



OF RESPIRATION. 



1 BY the air is meant that common elastic fluid which en- 

 velops the whole earth, and extends to a certain distance from 

 its surface. It constitutes what is called the atmosphere. By 

 its weight, its compressibility, and its pressure in all directions, 

 it insinuates itself into every vacuity ; and its presence is ab- 

 solutely necessary to the existence of every vegetable and ani- 

 mal. In order, however, to understand the manner in which 

 it contributes to the support of living things, it is necessary 

 to give some account of its composition. 



' Although the air, as we breathe it, seems to be a simple 

 and homogeneous fluid, yet it is in fact composed of two dis- 

 tinct constituent or elementary parts, upon the mixture or 

 combination of which, its adaptation to the preservation of life 

 depends; containing, besides, some other ingredients of minor 

 importance. These main elements are two permanently elas- 

 tic fluids or gases, called oxygen, and nitrogen or azote. At- 

 mospheric air contains about twenty-three parts, by weight, of 

 the former, and seventy-seven of the latter, out of one hun- 

 dred ; or, since oxygen is the heaviest of the two gases, twen- 

 ty-one, by measure of oxygen, and seventy-nine of azote. It 

 is upon the oxygen of the air, that its fitness for supporting 

 animal life depends; for, when an animal is confined in a 

 small quantity of air till this is exhausted, it dies from suffoca- 

 tion, although the azote remains unaltered. 



' No animal can exist in an active state without air, but dif 

 ferent classes of animals differ very much as to the manner in 

 8 



