78 



BRITISH PLANTS 



1. Oxygen. 



Los8 to the Air. 



Oxygen is withdrawn from the 

 air, and chemically fixed in other 

 substances in various ways. The 

 process is called combustion, or oxi- 

 dation, the chief modes of which 

 are: 



1. Burning, as by fire (rapid 

 combustion). 



2. The oxidation or rusting of 

 metals (slow combustion). 



3. Respiration, or breathing, both 

 in animals and plants (physiological 

 combustion). 



4. Putrefaction and decomposi- 

 tion, entailing the absorption of 

 oxygen (organic combustion). 



Oain to the Air. 



Oxygen is restored to the air 



by: 



Green plants 

 in the presence of light withdraw 

 carbonic acid gas from the air, 

 retain the carbon, which they build 

 up into carbohydrate, and restore 

 again to the air the oxygen not 

 required. 



The gain balances the loss. 



2. Carbonic Acid Gas. 



Loss to the Air. 



Carbonic acid gas is extracted 

 from the air in photosynthesis. 



Oain to the Air. 



It is added to the air during the 

 destruction of organic material. 

 All organic substances are com- 

 pounds of carbon. They are de- 

 stroyed by combining with oxy- 

 gen, being broken into simpler 

 bodies, one of which is always 

 carbonic acid gas : 



1. Destruction by slow com- 

 bustion, as in decomposition or 

 decay. 



2. Destruction by rapid com- 

 bustion, as in the burning of wood 

 and coal. 



3. Destruction by physiological 

 combustion, as in the respiration 

 of plants and animals. 



The gain balances the loss. 



