42 THE MECHANICAL ANALYSIS OF SOILS [chap. 



practically no quartz particles, all the silica being in 

 combination with the bases. 



A more detailed examination of the analyses of the 

 clay fractions suggest that two distinct forms of clay may 

 occur, the one from comparatively fertile soils contain- 

 ing about 21 per cent, of alumina and 51 to 54 per cent. 

 of silica, the other, from more infertile clay soils, con- 

 taining up to 30 per cent, of alumina and less than 50 

 per cent, of silica. The data, however, are as yet scanty. 



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 

 which are less than one-fivehundredth of a millimetre in 

 diameter. 



But though fineness of grain is a factor, it is probably 

 not the only factor ; the fine particles must be of such 

 a composition as to form a colloid system with water. 

 This may be illustrated by a consideration of another 

 important property of clay its power of flocculating 

 or coagulating under the action of minute quantities 

 of various salts. To illustrate this point, a few grams 



