36 EARTHS AND SOILS. 



essential in both the animal and vegetable structure, they 

 are not considered as forming any portion of the proper 

 food of either. Lime enters adventitiously into the food 

 of animals, and is transformed into bone. Silex enters 

 in the same way into the food of vegetables, and forms a 

 part of the epidermis of plants, like those we have named, 

 rendering them hard and rigid ; and seems designed to 

 strengthen and defend them from the attacks of insects 

 and parasitical plants. The earthy parts of the soil are 

 useful in retaining water, an essential agent in preparing 

 the food of vegetables, and the medium of conveying the 

 food thus prepared into and through the vegetable struc- 

 ture ; and they are also useful in producing the proper 

 distribution of animal and vegetable matter. It is the 

 finely-divided matter, principally clay and lime, which 

 gives tenacity and coherence to soils, a strong affinity for 

 moisture and manures, and which most tends to fertility, 

 when it does not exist in excess. 



" A certain degree of friability, or looseness of tex- 

 ture, is also required in soils, in order that the operations 

 of culture may be easily conducted ; that moisture may 

 have free access to the fibres of the roots ; that heat 

 may be readily conveyed to them, and that evaporation 

 may proceed without obstruction. Both water and air 

 must circulate in a soil, to render it productive. Hence 

 the presence of sand is necessary. As alumina possesses 

 all the properties of adhesiveness in an eminent degree, 

 and silex those of friability, it is obvious that a mixture 

 of these two earths, in suitable proportions, would furnish 

 every thing wanted to form the most perfect soil, as to 

 water and the operations of culture. In a soil so com- 

 pounded, water will be presented to the roots by capillary 

 attraction. It will be suspended in it, in the same man- 

 ner that it is suspended in a sponge, not in a state of 

 aggregation, but minute division, so that every part may 

 be said to be moist, but not wet." — Grisenthwaite. 



Another property to be regarded as of value in a soil, 

 is its capacity to absorb moisture from the atmosphere, 

 in which vapors more or less always abound. The soils 

 which possess this property in the highest degree, are 



