AGRICULTURAL TEXT-BOOK. 



the light, absorb carbonic acid, decompose it, retain the carbon, 

 and emit an equal volume of oxygon. 



Plants thus improve the air by the removal of carbonic acid nnd by 

 by the renewal of oxygen, \vhich is immediately applied to the use of 

 man and animals. The horizontal currents of the atmosphere bring with 

 thorn as much as they carry away, and the interchange of air between 

 the upper and lower strata, which their difference of temperature causw&amp;gt;, 

 is extremely trifling when compared with the hoiizontal movements of 

 the winds. Thus vegetable culture heightens the healthy state of a 

 country ; and a previously healthy country would be rendered quite un 

 inhabitable by the cessation of all cultivation. 



47. But in the dark, or during the night, the reverse action 

 takes place. Then green plants emit carbonic acid and absorb 

 OKygen ; while the whole volume of air undergoes diminution at 

 the same time. From the latter fact it follows that the quanti 

 ty of the oxygen gas absorbed is greater than the volume of 

 carbonic acid seperated ; for if this were not the case no dimi 

 nution could occur. 



42. Liebig has endeavored to prove that plants absorb di 

 rectly from the atmosphere much of the nitrogen (ammonia,} 

 which they require in their composition ; but his views are now 

 generally considered unsound ; and the ablest physiologists teach 

 that plants in a normal state imbibe carbonic acid and water 

 only through the leaves, while all other constituents enter by the 

 roots through the soil. Mulder lays down the following princi- 

 pie as sufficiently established : It is the function of the roots 

 to convey to plants water, ammonia, organic salts, and a small 

 quantity of inorganic salts; but that of the green parts, espe 

 cially of the leaves, to increase the amount of the non-nitroge 

 nous constituents of plants, by the absorption of carbonic add, 

 accompanied with an emission of oxygen. 



43. The other components of the atmosphere must, therefore, 

 reach the plant through the soil ; they being generally impart 

 ed to the soil by rain or snow. Their value and importance 



