306 PHYSICAL GEOGRAPHY. 



that variations which microscopic animals may occasion in 

 the quantity of oxygen held in solution in the water may entail 

 corresponding variations in the strata of air immediately in 

 contact with it. ( 3 ?7) The air which Martins collected on the 

 Faulhorn at a height of 8226 Trench feet had not less 

 oxygen than the air at Paris. ( 378 ) 



The admixture of carbonate of ammonia in the atmosphere 

 may have been older than the presence of organic life on the 

 globe. The sources from whence the atmosphere may derive 

 carbonic acid are manifold. ( 379 ) We may name first the 

 respiration of animals, who obtain the carbon which they 

 exhale from vegetable food, the vegetables receiving it from 

 the atmosphere. Other sources are, the interior of the earth 

 in districts of extinct volcanoes and thermal springs ; and the 

 deopmposition of the small portion of carburetted hydrogen 

 existing in the air by the electric discharges of clouds, which 

 are much more frequent within the tropics than in our 

 climates. With the above-mentioned substances, which we 

 find to be proper to the atmosphere at all elevations within 

 our reach, are accidentally associated others, especially in 

 dose vicinity to the ground, which, as miasma and pesti- 

 lential emanations, sometimes exercise a dangerous influence 

 on animal organization. Although the chemical nature of 

 these has not yet been ascertained by direct analysis, yet the 

 existence of such deleterious local admixtures may be 

 inferred, both from a consideration of the processes of decay 

 which the vegetable and animal substances with which our 

 globe is covered are perpetually undergoing, and from combi- 

 nations and analogies belonging to the domain of pathology. 

 Ammoniacal and other vapours containing azote, sulphuretted 

 hydrogen, and even combinations analogous to the ternary 



