78 BRITISH 
PLANTS 
1. Oxygen. 
Loss 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Oxygen is withdrawn from the 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 decompost- 
tion, entailing the absorption of 
oxygen (organic combustion). 
Gain to the Air. 
Oxygen is restored to the air 
by: 
_ Photosynthesis. 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. 
Gain 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.” 
