I 4 SCIENTIFIC AGRICULTURE. 



15. The elements of a soil vary with the kind of 

 solid rock from which it was originally formed. A 

 disintegrated sandstone, or limestone, or slate, or 

 granite, will each produce a soil of peculiar cha'rac- 

 ter. Very often the soil rests upon the rock from 

 which it was formed. Sometimes, however, it has 

 been carried away and deposited at a long distance 

 from the parent rock. Such deposits form what are 

 called alluvial soils, and are generally found in creek 

 and river bottoms (2). 



1 6. The great difference in the quality and value 

 of soils results mainly from a difference in the rela- 

 tive quantity of certain elements present. The total 

 absence of any of the essential elements of plants is 

 rare. Notwithstanding the great variety of mineral 

 and vegetable substances, the whole mass of the 

 earth and everything upon it is composed of few 

 elements, or simple substances (3). 



17. The number of elements certainly known is 

 sixty-five; several others have been announced 

 recently, but their existence is yet in doubt. Five 

 of these, oxygen, hydrogen, nitrogen, chlorine, and 

 fluorine, are gases, and two, bromine and mercury, 

 are liquids at the ordinary temperature of the air, 

 while all the others are solid. By reducing the 

 temperature and applying great pressure, the gases 

 can be changed to liquids, and the liquids can be 

 converted into solids. There are no permanent 

 gases ; even oxygen, hydrogen, and nitrogen have 

 been made to assume the liquid form. 



