THE SOIL 13 



in remaining suspended and the proportion which settles down 

 quickly, and attributes many of the typical clay properties 

 to the jelly-like medium of colloidal matter by which the other 

 denned particles of the clay are surrounded. Later researches, 

 however, show that the colloidal particles are not essentially 

 different except that they are exceedingly fine in texture. 



" Fineness of grain is an essential factor, the characteristic 

 properties of clay not being developed except in material the 

 particles of which are less than one five-hundredth part of a 

 millimetre in diameter. 



" But, although fineness of grain is a factor, it is probably 

 not the only factor, as may be seen from 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 grains of good clay should 

 be rubbed up with several litres of distilled water and the 

 turbid liquid poured off into several tall glass jars. To one of 

 these jars nothing should be added, to two others .018 and 

 .009 gram of hydrochloric acid respectively, to a fourth jar 

 .028 gram of calcium chloride, and to the fifth .058 gram of 

 sodium chloride. The contents of the jars should be shaken 

 up till solution is effected and they then should be put aside 

 to stand. After some time the liquid to which the salts have 

 been added will begin to clear and the clay particles will clot 

 together and fall to the bottom. The jar containing the larger 

 quantity of hydrochloric acid will clear first, the others will 

 clear approximately together, but the jar containing the pure 

 water and clay will remain turbid for many days. If a little 

 of the turbid clay water be examined under the microscope it 

 is just possible to see the clay particles in rapid " Brownian " 

 motion, and if a little salt or acid be then introduced under the 

 cover glass, the particles will be seen to move together and form 

 into little clots or aggregates as soon as they experience the 

 effects of the added acid or salt. 



" This aggregating together of the fine particles, which is 

 known by the term of ' flocculation/ may be aided by the use 

 of certain artificial manures. The incorporation of humus 

 or green manures much improves the texture, while the action 

 of lime is particularly effective and is much employed in practice 

 to ameliorate the working of clay soils. 



