AND EXTERNAL RESISTANCE. 319 



and herbivarous animals in particular, would 

 be suffocated and poisoned as effectually as if 

 they were exposed to the vapour of the grotto 

 del Cane, or of the lake of Averno. A benefi- 

 cent Providence has directed it otherwise : the 

 weight of different gases is entirely overcome, 

 by the equal degrees of expansible power 

 which they separately possess. 



If the chemical properties of these gases are 

 examined, they will be found very different 

 from each other. The properties of nitrogen 

 would seem to be rather of a negative, than of 

 a positive nature; it is neither absorbed by 

 pure water, in any sensible quantity, neither 

 does it render lime-water turbid, or turn it 

 white, it produces no change in vegetable 

 color, and is totally unable either to support 

 combustion, or subserve to the process of re- 

 spiration. 



Oxygen, on the contrary, is the principal 

 agent in the production of combustion, and 

 when mixed in the proportion in which it is ge- 

 nerally found to exist in the atmosphere, is best 

 adapted to promote, and to subserve to the pro- 

 cess of respiration. It is obtained from differ- 

 ent substances, by factitious and unnatural 

 means ; it is disengaged by the operation of an 

 ardent fire, from nitre, from manganeze, and 

 other bodies, when closely confined in a retort. 

 It is very generally excreted during the day by 



