WEATHERING OF ORES. 120 



The means of obviating this sterility follows from a knowledge 

 of its cause. 



Empiricism ascribes all results to the art, that is, to the me- 

 chanical operations employed in cultivation, without inquiring 

 the causes upon which their use depends. But a knowledge of 

 these causes is of the highest importance ; for such knowledge 

 would prevent the lavish expenditure of capital and of power, 

 and would enable us to use them in the most advantageous man- 

 ner. Is it conceivable that the entrance of the ploughshare or 

 of the harrow into the earth — that the contact of iron with the 

 soil — can act as a charm to impart fertility 1 No one can enter- 

 tain such an opinion ; and yet the causes of their action have not 

 yet been inquired into, and much less have they been explained. 

 It is quite certain that it is the great mechanical division, the 

 change and increase of surface, obtained by the careful plough- 

 ing and breaking up of the soil, which exercises so very favora- 

 ble an influence on its fertility ; but these mechanical operations 

 are only the means to attain that end. 



Among the effects produced by time, particularly in the case 

 of fallow, or that period during which a field remains at rest, 

 science recognises certain chemical actions, which proceed unin- 

 terruptedly by means of the influence exercised by the constitu- 

 ents of the atmosphere upon the surface of the solid crust of the 

 earth. By the action of the carbonic acid and oxygen in the air, 

 aided by moisture and by rain-water, the power of dissolving in 

 water is given to certain constituents of rocks, or of their debris, 

 from which arable land is formed ; these ingredients, in con- 

 sequence of their solubility, become separated from the insoluble 

 constituents. 



These chemical actions serve to explain the effects produced 

 by the hand of time, which destroys human structures, and con- 

 verts gradually the hardest rocks into dust. It is by their influ- 

 ence that certain ingredients of arable land become fit for assimi- 

 lation by plants ; and the object of the mechanical operations of 

 the farm is to obtain this result. Their action consists in acce- 

 lerating the weathering or disintegration of the soil, and thus 

 offers to a new generation of plants their necessary mineral con- 

 stituents, in a form fit for reception. The celerity of the disin- 



