100 NATURE TEACHING. 



is commonly well exhibited in sugar, blotting- 

 paper, arid similar porous substances. If one 

 of these is gently brought into contact with 

 a drop of water, the water enters the small 

 pores or cavities and spreads over a large 

 area where it is retained and from which 

 it will not drain away again. By means of 

 this power soils retain a sufficiency of water 

 for the use of plants, the small spaces being 

 filled with water while the larger spaces con- 

 tain air. The soil is thus provided with both 

 of these requisites for plant growth. 



CLAY. 



1. It has already been said that clay is 

 formed from felspar by the action of water and 

 carbon dioxide. Pure clay consists of extreme- 

 ly minute particles, but soils are never pure 

 clay, there being always a certain amount of 

 sand present. The fine particles of clay have 

 a tendency to collect together in groups or 

 masses. If this were not the case all the 

 small openings and passages in the soil would 

 be choked and drainage rendered impossible. 

 Clay also has the power of absorbing water 

 and becoming plastic, that is to say, it can be 

 kneaded and moulded by the hand, a property 

 which is taken advantage of in the making of 



