30 THE PRINCIPLES OF 



Las been left as a plastic mass, seen in its purest forms as 

 porcelain clay, white bole, and pipe-clay. Viewed chemically, 

 silicate of alumina cannot be spoken of as a plant food in any 

 sense whatever. The silica is in chemical combination with 

 the alumina, and alumina never enters into the composition 

 of, and is never wanted within, the plant. The silicate of 

 alumina or clay does not take any direct part in the nutrition 

 of plants. 



Nevertheless, a clay soil is usually a rich soil. It is diffi- 

 cult to work, and has been under a cloud of late years on 

 account of the low price of corn. Still clay must be considered 

 a guarantee of richness in a soil. For why ? It is because 

 in nature substances rarely occur in a state of chemical 

 purity. There are always impurities associated with them, 

 and such is the case with clay. Pure clay is white, but 

 agricultural clay is red, the redness being due to a very 

 important element of plant-food iron. The redness of clay 

 betrays at once the presence of oxide of iron. In blue 

 clay there is an admixture of vegetable matter, or it may 

 be of lower oxides of iron protoxides or protosalts, which 

 speedily are peroxidized when they are exposed to the action 

 of the atmosphere. 



Clays are frequently described as marls the term indicat- 

 ing the presence of lime. We shall not find any agricultural 

 clay destitute of lime. One of the great difficulties in making 

 bricks and tiles is the presence of lime in the form of nodules, 

 and it is scattered through the mass in greater or lesser 

 quantities. We may expect to find lime in clays, and with 

 it the acids which combine with lime. For instance, nitrate 

 of lime, sulphate of lime, carbonate of lime, and phosphate 

 of lime. Clays also usually contain magnesia ; and they 

 are invariably rich in potash. It is "the silicate of potash 

 which gives that peculiar soapy feeling characteristic of wet 

 clay. So, while clay is itself perfectly barren and incap- 



