36 AGRICULTURE. 



the residuum of dead animal or vegetable matter,* 

 they become fertile, take the general name of soils , 

 and are again specially denominated after the earth 

 that most abounds in their compositions respective- 

 ly. If this be silica, they are called sandy ; if alu- 

 mina, argillaceous ; if lime, calcareous ; and if mag- 

 nesia, magnesian. Their properties are well known : 

 a sandy soil is loose, easily moved, little retentive 

 of moisture, and subject to extreme dryness ; an ar- 

 gillaceous soil is hard and compact when dry, tough 

 and paste-like when wet, greedy and tenacious of 

 moisture ; turns up, when ploughed, into massive 

 clods, and admits the entrance of roots with great 

 cdHblty. A calcareous soil is dry, friable, and po- 

 rBp water enters and leaves it with facility ; roots 

 penetrate it without difficulty, and [being already 

 greatly divided] less labour is necessary for it than 

 for clay. Magnesian, like calcareous earth, is light, 

 porous, and friable^ but, like clay, when wet, takes 

 the consistency of paste, and is very tenacious of 

 water. It refuses to combine with oxygen or with 

 the alkalies : is generally found associated with 

 granite, gneiss, and schist, and is probably among 

 the causes of their comparative barrenness.! 

 quartz, alumina, and sometimes of calcareous matter. Specu- 

 lative geology is romance, and does not merit the name of sci- 

 ence ; yet is science obliged to borrow her theory of soils. The 

 alternation of heat and cold, moisture and dryness, decomposed 

 the mountains of primitive, secondary, and tertiary formation ; 

 rains, and the laws of gravity, brought these broken parts from 

 places of more to those of less elevation ; where, by mechani- 

 cal mixture and chymical combination, the present substrata 

 were formed. But these were yet naked and unproductive, 

 when the Cryptogamia family (mosses and lichens) took pos- 

 session of them, and in due time produced that vegetable matter 

 which made the earth productive and the globe habitable ! 



* Dead animal and vegetable matter, in the last stage of de- 

 composition, give a black or brown powder, which the French 

 chymists call terreau or humus, and which Mr. Davy calls an ex- 

 tractive matter ; this is the fertilizing principle of soils and ma- 

 nures. 



t The opinion is general among the chymists of Europe, that 



