EARTHS AND SOILS. 101 



As a full description of all the distinctive characters 

 of these earths could not be embraced within the com- 

 pass of my contemplated communication, I shall mere- 

 ly mention a few, by which may be understood the 

 effects they necessarily produce in forming a compound 

 best calculated to produce vegetation. 



Silicious earth exists in the state of sand. It is 

 composed of very hard particles, which cannot be 

 made coherent by mixing with water. The solidity 

 of the particles of sand renders them impenetrable to 

 water, and their loose arrangement makes sand incapa- 

 ble of retaining water. It is also quickly heated by 

 the sun, which adds to the rapidity with which it 

 loses moisture. 



Aluminous earth, when dry, adheres to the tongue, 

 absorbs water rapidly and abundantly, and, when wet, 

 forms a tough paste, smooth and soapy to the touch. 

 By burning, it becomes very hard : when drying, 

 aluminous earth shrinks greatly ; it becomes a mass of 

 hard lumps, separated by cracks and fissures, which 

 become so many little reservoirs of standing water 

 when filled by rains, and remain so when the lumps, 

 by slowly imbibing the water, are distended enough 

 to fill the space occupied before. 



Calcareous earth, or carbonate of lime, is calcareous 

 earth combined with carbonic acid, and may be con- 

 verted into quicklime by heat. Lime is soluble in 

 acids: during the solution, the carbonic acid escapes 

 with effervescence. In this manner, the carbonic of 

 lime, or calcareous earth, may not be easily distin- 

 guished from silicious and aluminous earths, but from 

 all other combinations of lime. Magnesia earth, like 

 lime, is usually found in combination with carbonic 

 acid ; but even in this, its natural state, it exists in 

 such very small quantities in soils, and is found so 

 rarely, that its name is a useless addition to the lists of 

 the earths of agriculture. 



All the earths individually, when as pure as they 



