68 THE SOIL. 



depends, as in charcoal, upon a surface attraction, wliich 

 is termed a physical attraction, because the attracted 

 particles enter into no chemical combination, but retain 

 their chemical properties.* 



The arable soil owes its formation to the disintegration 

 of minerals and rocks, brought about by the action of 

 mighty mechanical and chemical agencies. Though tlie 

 comparison may not be altogether apt, the rock may be 

 said to stand in about the same relation to the arable soil 

 resulting from its disintegration as the wood or the vege- 

 table fibre to the humus residtmg from its decay. 



The same causes wliich in the course of a few years 

 convert wood into humus act also upon rocks, with this 

 difference, however, that it requues the combined action 

 of water, oxygen, and carbonic acid, for probably a 

 thousand years, to produce from basalt, trachyte, felspar, 

 or porphyry, the thinnest layer of arable soil (such as is 

 found in the plains of river valleys and low lands) with 

 all the chemical and physical properties suited for the 

 nutrition of plants. Sawdust possesses the properties of 

 humus no more than powdered rocks have the properties 

 of arable soil. ISTo doubt sawdust may pass into humus 

 and powdered stones into arable soil, but the two states 

 are essentially distinct ; and no human art can imitate the 

 operations which were necessary, during immense ages, 

 to convert the divers kinds of rocks into arable soil. 



Arable soil, resulting from the disintegration of various 

 kinds of rocks, bears the same relation, in respect of 

 absorptive power for inorganic substances in solution, as 

 the woody fibre altered by the action of heat bears to 

 organic substances in solution. 



* The term, ' physical attraction,' as used here, does not signify a 

 peculiar attractive force, but merely designates the ordinary chemical 

 affinity, which shows differences of degree in its manifestation. 



