NOURISHMENT OF PLANTS. 35 



etc. Lastly, it streams forth from crevices in many 

 regions of the earth where volcanic energies are ac- 

 tive, or, as we may conjecture, were active in a for- 

 mer age. 



All the carbonic acid generated by these different 

 processes is taken up into the air. If it should con- 

 tinue there, the air must of necessity become gradu- 

 ally deteriorated and unsuitable for respiration ; more 

 especially as in all the processes of breathing, com- 

 bustion, and decay, free oxygen or vital air is re- 

 moved from it. But this is not the case. The oxy- 

 gen does not decrease, the carbonic acid does not 

 increase. The vegetable world discharges the func- 

 tion, not only of a supporter, but also of a protector 

 of animal life. It does not merely provide the whole 

 animal kingdom with nourishment, but also restores 

 again to the air the oxygen abstracted by the former. 

 For plants by their roots and leaves absorb carbonic 

 acid as their most important article of nourishment, 

 and by their green or herbaceous parts again exhale 

 its oxygen during the light of day. On the other 

 hand, they firmly retain the carbon of the carbonic 

 acid, and appropriate it to the construction of their 

 leaves, blossoms, seed, and the proximate constit- 

 uents which these contain. 



Carbonic acid is generated in the soil wherever 

 plants are produced. The falling leaves, the roots 

 remaining in the ground, and the worms and insects 

 which feed thereon, all become, as soon as life has 

 left them, the subjects of corruption and decay. By 



