38 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Smith found the air in Glasgow to contain 0.05 per cent, and in 

 London 0.044 per cent. More carbon dioxide is present in the 

 air at night than in the day. For example, Armstrong 1 found 

 the air to contain 0.0296 per cent, carbon dioxide in the day, and 

 0.330 per cent, at night. 



CARBON D/OX/DE OF Am 



Off&AN/C 



GARBONOF 

 PLANTS 



Fig. 5. Circulation of carbon. 



These variations are easily explained. In cities more carbon 

 dioxide is evolved by respiration and combustion than in the 

 country, and less is absorbed by plants. At night the production 

 of carbon dioxide by animals continues, but plants give it off 

 instead of absorbing it, hence the larger amount at night. 

 1 Proc. Roy. Soc., 1880, p. 343. 



