CARBONIC ACID OxAS.

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6.3. Hence we see that decay is very like burning,similar effects being produced by both ; only that thechange which is effected by combustion in a short time,and accompanied with a great deal of heat, is veryslowly and gradually effected by decay, very little heatbeing at the same time evolved. In both cases carbonicacid gas is produced, which is essential to the growthof plants, as they, being able to decompose it, thusobtain carbon.

64. This explains the use of decaying vegetable sub-stances in manures; they, of course, contain carbon,which is slowly combining with oxygen, and therefore,always supplying the growing plants with carbonic acidgas (29).

65. Carbonic acid gas is produced in large quantityby the breathing of animals. A constant supply offresh air is requisite for the support of life, and weknow that if an animal is prevented from breathing orinhaling fresh air, it will very soon be suffocated. Thechemical action which goes on in the lungs of ananimal is just the reverse of that which takes place inthe organs of plants: in this case carbonic acid gas inthe air is decomposed, and the carbon which it con-tained is appropriated by the plant; whilst in thelungs of an animal carbonic acid is formed ; for theoxygen of the air is found, on being expelled from thelungs, to have combined with carbon and become con-verted into carbonic acid gas. In fact the process of