I.] INORGANIC MATTER OF SOILS. 15 



reference to the fertility of the land. Clay is chiefly 

 composed of silicate of alumina, that is to say, 

 silica combined with alumina. The fertility of clay, 

 however, is very largely dependent upon the presence 

 of a peculiar form of silicate of alumina, which de- 

 mands special notice. 



24. Some few years since Professor Way carried out 

 an investigation into the character of the silicates of 

 alumina, and disclosed truths of immense importance 

 which have not been generally understood, and conse- 

 quently have not been taken advantage of. He 

 showed the existence of a class of bodies which are 

 termed double silicates. These were silicates of 

 alumina in which part of the alumina had been replaced 

 by an equivalent quantity of some other substance, 

 such as lime, soda, potash or ammonia. Thus we 

 have these double silicates in soils as silicate of 

 alumina and lime, as silicate of alumina and soda, as 

 silicate of alumina and potash, or as silicate of alumina 

 and ammonia. These substances will hereafter be seen 

 to be exceedingly important, and a familiar acquaintance 

 with them is most desirable. 



The Alumina also possesses in one respect an 

 exceptional character. Whilst it is most valuable in 

 assisting other bodies to enter into the crops growing 

 upon the land, it appears to act rather as an " out-of- 

 door servant," carefully avoiding going into the plant. 



25. Phosphoric acid is by general consent con- 

 sidered as one of the most important substances found 

 in the soil. Its influence upon the fertility of the land 

 is very great, for every cultivated plant requires a 

 supply for its successful growth. In combination with 

 lime it forms a large portion of the skeletons of animals, 

 and the demand which cultivated plants thus make 

 upon the soil for phosphoric acid enables them more 

 perfectly to supply the requirements of the animals by 

 which these are afterwards used in food. A supply of 

 phosphoric acid in the soil is therefore of very great 



