AGRICULTURAL TEXT-BOOK. 



the acquisition of knowledge and by intelligent labor, so to mo 

 dify existing circumstances as to cause a plant to grow profitably 

 where naturally it could not do so. 



23. On those simple principles, depends the utility of plow 

 ing, harrowing, manuring, irrigation, and the other mechanical 

 processes of cultivation. 



24. In other words, the art of Agriculture, when confined to 

 growing plants, is simply adapting the soil, and surrounding ci*- 

 omnstances to the natural demands of the plant; each plant 

 rarying somewhat from another, in its requirements. 



25. In order to do this, it is necessary to understand, (a,) tie 

 materials which are or can be supplied by air, water, and soil; 

 (for in this connection, manure is merely a modification of the 

 mil,) (b,) the materials which enter into the composition of each 

 plant; and (c,) the incidental changes, which meteorology and 

 chemistry may produce. 



26. THE AIR or ATMOSPHERE. According to M. Regnault, 

 100 cubic inches of atmospheric air, deprived of aqueous vapor 

 and carbonic acid, weigh 30.82926 grains. At 62 Fahr. air 

 is 810 times lighter than water, and 11,000 times lighter than 

 mercury. 



The chemical composition of air is, (Dumas and Boussin- 

 yaxdt) : 



Air by Weight. Air by Volume. 



100.00 100.00 



Besides these constituents, the atmosphere always contains a 

 variable quantity of watery vapor and carbonic acid gas, besides 

 other gasses and vaporous bodies which are observed to enter 

 into it The younger Saussure has ascertained that the mean 

 : proportion of carbonic acid is 4.9 volumes in 10,000 volumes of 

 air; or almost exactly 1 in 2,000 volumes; but it varies from 

 8.2 as a maximum to 3.7 as a minimum, in 10,000 volumes. 



