ATMOSPHERE ASI> CLIMATE, ETC. 11 



work, or it may be used in the living body to produce the 

 energy necessary to pump the blood, move the muscles, or 

 build up new tissues. The heat produced in decaying vege^ 

 table matter is another manifestation of slow combustion. 

 Sometimes, in manure containing very much vegetable 

 matter, so much heat is produced by slow combustion that 

 the heap is able to break into fire or rapid combustion. 

 Enough has been said to show that the heat used to drive 

 the engine, the heat supporting the internal life of animals 

 and plants, and the heat arising from decaying vegetable 

 matter, is produced in each case from a similar cause, 

 namely the oxidation of organic matter. In such cases 

 therefore oxygen is necessary for the production of heat 

 and vital energy, and carbon dioxide is a product resulting 

 from the oxidation of organic substances. 



Animals and plants exhale carbon dioxide and -fires 

 give off this gas, yet, except in enclosed spaces, it never 

 becomes appreciably greater in quantity in the atmosphere. 

 This is because the carbon dioxide is a plant food and is 

 constantly being extracted from the air to build up the 

 plant body. When air enters the plant, the carbon dioxide 

 is split up, the oxygen going back to the atmosphere 

 while the carbon is used to build up the solid plant body. 

 What plants take from the air i. e. carbon dioxide is 

 returned by breathing, burning and decay : what animals 

 take from the air oxygen is returned by plants. The 

 materials which plants take from the soil, are returned 

 again in the dead bodies and excreta of animals and 

 plants, and the residue of fires. Animals and plants form 

 one cygle capable of living and taking the materials for 



