78 



BRITISH PLANTS 



1. Oxygen. 



Loss to the Air. 



Gain to the Air. 



Oxygen is withdrawn from the Oxygen is restored to the air 

 air, and chemically fixed in other ; by : 



substances in various ways. The Photosynthesis. Green plants 

 process is called combustion, or oxi- j in the presence of light withdraw 

 dation, the chief modes of which carbonic acid gas from the air, 

 are : ' retain the carbon, which they build 



1. Burning, as by fire (rapid | up into carbohydrate, and restore 

 combustion). ' again to tlje air the oxygen not 



2. The oxidation or rusting of required, 

 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). 



The gain balances the loss. 



2. Carbonic Acid Gas. 



Loss to the Air. 



Gain to the Air. 



Carbonic acid gas is extracted | It is added to the air during the 

 from the air in photosynthesis. 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. 



