CARBONIC ACID GAS. 65 



exposed to the air, it usually happens that, if the 

 other elements which it contains combine with oxy- 

 gen, the carbon also is then able to combine with 

 oxygen, and forms carbonic acid gas. 



104. Thus when a plant dies and decays, its ele- 

 ments separate and form new compounds ; but the 

 carbon is not set free in the form of black charcoal 

 — it is slowly combined with oxygen to form carbonic 

 acid gas (767). 



105. Hence we see that decay is very like burning, 

 similar effects being produced by both; only that the 

 change which is effected by combustion in a short 

 time, and accompanied with a great deal of heat, is 

 very slowly and gradually effected by decay, very 

 little heat being at the same time evolved. In both 

 cases carbonic acid gas is produced. 



106. This explains one great use of decaying vege- 

 table substances in manures; they, of course, con- 

 tain carbon, which is slowly combining with oxygen, 

 and therefore always supplying the growing plants 

 with carbonic acid gas, which is essential to their 

 growth, as they, being able to decompose it, thus 

 obtain carbon (698, 708). 



107. Carbonic acid gas is produced in large quan- 

 tity by the breathing of animals. A constant supply 

 of fresh air is requisite for the support of life, and 

 we know that if an animal is prevented from breath- 

 ing or inhaling fresh air, it will very soon be suffo- 

 cated. The chemical action which goes on in the 

 lungs of an animal, is just the reverse of that which 



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