ENGLISH AGRICULTURE. 31 



able of supporting vegetation, as it is associated with these 

 substances it becomes an index to the fertility of the 

 soil. 



Clay is then associated with various substances which 

 confer wealth or fertility upon soil, but its mechanical or 

 physical properties are also very important. Clay is highly 

 retentive. Not only can it hold a large volume of water, but 

 it retains it with great tenacity. It also condenses gaseous 

 matter upon its surfaces, by which we mean its internal 

 superficies, not its external surface. It exerts the power which 

 is possessed by substances in a very minute state of division. 

 The amount of surface in a cubic inch of clay is vast, for the 

 smaller or finer the state of division in a substance, the 

 greater the amount of its surface. 



Clay parts with its heat quickly, and receives heat slowly, 

 so that clay soils are inclined to be wet and cold. The 

 expression " cold clay " is quite usual in speaking of clay 

 land. 



Clay is one of the proximate constituents of all fertile soils. 

 Bereft of clay, a soil would be useless for agricultural purposes. 

 Take the clay out, and we should have a blowing sand, and 

 we should deprive the soil of a large number of its principal 

 mineral constituents. 



In the next place, we turn our attention to sand. 

 Viewed chemically, sand is as unpromising as clay. We 

 must however clearly define what we mean by sand. First, 

 there is silicious or quartzoze sand, composed of particles of 

 quartz or silicic acid. It is insoluble in water and acids, and 

 in the case of silver sand it is composed almost entirely of 

 insoluble silica. 



Sea-shore sand is generally coloured with oxide of iron, 

 but it is also associated with fragments of shell which confer 

 upon it a certain fertilizing value. Viewed purely as silicious 

 or quartzoze sand it is as entirely barren as clay. A mixture 



