18 OF THE ASSIMILATION OF CARBON. 



carbonic acid, does not change its volume. The 

 billions of cubic feet of oxygen extracted from the 

 atmosphere, produce the same number of billions 

 of cubic feet of carbonic acid, which immediately 

 supply its place. 



The most exact and most recent experiments of 

 De Saussure, made in every season, for a space of 

 three years, have shown, that the air contains on an 

 average 0.000415 of its own volume of carbonic 

 acid gas ; so that, allowing for the inaccuracies of 

 the experiments, which must diminish the quantity 

 obtained, the proportion of carbonic acid in the 

 atmosphere may be regarded as nearly equal to 

 1-1000 part of its weight. The quantity varies 

 according to the seasons ; but the yearly average 

 remains continually the same. 



We have no reason to believe that this propor- 

 tion was less in past ages ; and nevertheless, the 

 immense masses of carbonic acid, which annually 

 flow into the atmosphere from so many causes, 

 ought perceptibly to increase its quantity from year 

 to year. But we find, that all earlier observers 

 describe its volume as from one-half to ten times 

 greater than that which it has at the present time ; 

 so that we can hence at most conclude, that it has 

 diminished. 



It is quite evident, that the quantities of carbonic 

 acid and oxygen in the atmosphere, which remain 

 unchanged by lapse of time, must stand in some 

 fixed relation to one another ; a cause must exist 



