44 THE MECHANICAL ANAL YSIS OF SOILS [chap. 



The alkalis and salts like phosphate of sodium, which 

 give rise to free alkalis on hydrolysis, instead of 

 flocculating have the opposite effect, and keep the 

 particles in their finest state of division without any 

 tendency to settle. 



In their behaviour towards flocculating salts the 

 finest particles of quartz and ferric oxide behave essen- 

 tially differently from kaolinitic particles. Suspended 

 in water they flocculate spontaneously and will not 

 remain turbid for many minutes, though in their turn 

 they can be deflocculated and made to remain in 

 suspension by adding a trace of free alkali to the liquid. 

 Soils are known composed of particles of great fine- 

 ness of grain, but destitute of colloidal properties, for 

 example, soils made from the fine quartz particles 

 " tailings and slimes," coming from the crushing plant 

 of a gold mine. These soils cannot be flocculated and 

 are extremely difficult to cultivate. 



We may thus conclude that fineness of grain is 

 not the only factor in the constitution of clay, but that 

 the characteristic clay properties which are always 

 associated with the power of flocculation depend also 

 upon the nature of the material; in the soil they depend 

 upon the presence of the zeolitic double silicates derived 

 from the weathering of the felspars in the fundamental 

 rocks. We may imagine the soil as consisting of 

 particles of all sizes down to the ultra microscopic. 

 Most of the particles possess a covering of a jelly-like 

 colloidal character which chemically resembles the 

 zeolites in that it is a silicate containing many bases 

 (alumina, calcium, magnesium, potash, soda, and 

 ammonia), which can be readily interchanged with other 

 bases in the soil solution. The finer the particles 

 become the thicker is the jelly layer in comparison, 

 until the finest, in which the colloidal properties are 



