56 AGRICULTURE 



when pure it contains nothing of value as 

 plant food, it always holds considerable 

 quantities of substances passing into the 

 form of clay, which contain materials like 

 potash that can be utilized in the nutrition 

 of plants. Just as sand may be denned 

 chemically as oxide of silicon, so clay may 

 be denned as a hydrated silicate of aluminium. 

 Bodies are said to be hydrated when they 

 contain water as an essential part of their 

 composition, and so long as clay is hydrated 

 it is plastic and sticky, and can be moulded 

 into any desired form. But the water of 

 hydration can be driven from clay by ex- 

 posure to a somewhat high temperature, 

 and then the substance is no longer clay. 

 Although it is still a silicate of aluminium, it 

 is not a hydrated silicate. This change takes 

 place where clay is burned in the process of 

 brickmaking, the main difference between a 

 brick and clay being that the former does 

 not contain water as an essential part of its 

 composition, whereas the latter does. 



When clay has been dehydrated it cannot 

 be restored to its original condition by any 

 ordinary process, so that one may grind a 

 brick into powder and mix it with water, 

 but one will not by so doing restore the 

 characters of clay. The same change is 



