160 



they prevent it from too rapid u decomposition ; and they 

 also supply tlic soluble parts in their due proportions. 



Kirwan, in his Geological Essays, has shown, that the 

 fertility of a soil in a great measure depends on its capacity 

 to retain water. The power of the soil to absorb water by 

 cohesive attraction, depends, in a considerable degree, on the 

 division of its parts. The more these are divided, the greater 

 their absorbent power. Hence the great importance of fria- 

 bility or looseness of texture ; so that moisture may have free 

 access to the fibres of the roots, that heat may be readily con- 

 veyed to them, and that evaporation may proceed without 

 obstruction. These benefits are usually attained by the 

 presence of sand. As alumina possesses, in an eminent de- 

 gree, all the powers of adhesiveness, and silex those of fria- 

 bility, it is obvious, that a mixture of those earths in suitable 

 proportions, would furnish every thing that could be wanted 

 in the most perfect soil. In a soil so constituted, water would 

 be presented to the roots by capillary attraction. It would 

 be suspended in it, says Griesenthwaite, in the same way as 

 in a sponge, that is, in a state not of aggregation, but of mi- 

 nute division, so that every part might be moist, but not 

 wet.* Hence the best soil, whether for wood or agricultural 

 crops, obviously is one that is at once loose and deep, con- 

 taining the most alumina and carbonate of lime, so as to act 

 with the greatest chemical energy, in the preservation of 

 manures.t 



Trees, far more than agricultural crops, require depth of 

 soil, to raise them to perfection: the effect of climate ap- 

 pears much less necessary, in giving them their greatest 

 magnitude. Accordingly, notwithstanding the insularity of 

 our situation, which naturally tends to the equalization of 

 climates, little park-timber is found in Scotland, or the north 

 of England, approaching in size and grandeur to the great 



* New Theory of Agriculture. f Note I 



