II.] 



THE NATURE OF CLA Y 



37 



or could easily be rubbed into a fine powder, only 

 the fourth and finest fraction dried into a hard co- 

 herent mass. Thus we can have material which con- 

 sists entirely of kaolinite, and yet is not clay ; such as 

 we see in natural deposits of fuller's earth, which 

 consists of kaolinite but possesses no plasticity, and 

 falls on drying into a fine powder. 



In the same way a natural soil contains particles 

 of silicates of alumina of all sizes, though they only 

 begin to predominate in the fractions of finest grain. 



On separating one of the Rothamsted soils into 

 fractions, according to their size by the method to be 

 described later, and analysing them, the following results 

 were obtained : — 



Taking the mean of several analyses, the fifth fraction, 

 which is to be regarded as clay proper, possessed the 

 following approximate composition, if all the alumina is 

 combined as A1 2 3 , 2Si0 2 , 2H 2 — kaolinite 72 to 75 

 per cent; ferric oxide, 11 to 12 per cent; quartz, 9 

 to 10 per cent; alkalis and alkaline earths, 4 to 6 per 

 cent. 



From these results we must conclude that kaolinite 

 is not necessarily clay, but that fineness of grain is also 

 an essential factor, the characteristic clay properties not 

 being developed except in material the particles of 



