16 PHYSICAL CONSTITUTION OF SOIL 



repeated. The clay water finally obtained also contains more 

 than one group of matter. Subsidence for twenty-four hours 

 will remove all particles of sand exceeding -005 mm. in dia- 

 meter, but much of a still finer nature will remain suspended 

 with the true clay. 



True colloid, jelly-like clay, forms according to Schloesing 

 a very small proportion of the clay obtained by the method 

 just described. To obtain the colloid clay he precipitates 

 the clay water with weak acid ; the clay is then collected 

 on a filter, and washed with distilled water. When removed 

 from the filter the clay is treated with a small quantity of 

 ammonia, and finally diffused in a considerable volume of 

 distilled water. The whole is then left for weeks or months 

 till all deposit ceases. The microscope can then no longer 

 detect particles of visible dimensions in the solution. The 

 colloid clay may then be precipitated by the addition of an 

 acid ; it dries to a translucent, horn-like mass. According 

 to Schloesing the true colloid clay forms seldom more than 

 i -5 per cent, of the stiffest natural clays. 



Besides colloid clay, the clay water contains the other 

 colloid bodies (hydrogels) present in the soil ; the commonest 

 of these are humic acid, humates, hydrated silicates (zeolites), 

 and hydrated ferric oxide ; hydrated silicic acid and hydrated 

 alumina are less frequent constituents. 



We must then always regard the clay in a mechanical 

 analysis of soil as a mixed group containing the finest 

 particles in the soil, but the precise nature of which will 

 vary in different soils, and will also depend on the method 

 of analysis adopted. 



The methods used for the mechanical analysis of soil give 

 their sharpest results with soils containing but little humus. 



