38 THE MECHANICAL ANAL YSIS OF SOILS [chap. 



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, 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 

 grams of good clay should be rubbed up with several 

 litres of distilled water, and the supernatant turbid liquid 

 poured off into a series of tall jars each holding from 

 300 to 500 c.c. of the liquid. To one of these jars 

 nothing is added, to two others -018 and 0-009 gram of 

 hydrochloric acid respectively, to a fourth 0-028 gram of 

 calcium chloride, and to the fifth 0-58 gram of sodium 

 chloride. The contents of the jars are shaken up until 

 solution is effected, and they are then put aside to stand. 

 After some time the liquids 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 the first, 

 the others will clear approximately together, but the pure 

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

 of the turbid clay water be examined by a T V-inch 

 oil immersion lens under the microscope, it is just 

 possible to see the clay particles in rapid " Brownian " 

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

 under the cover glass, they will be seen to move together 

 and form into little clots or aggregates as soon as they 

 experience the effect of the added acid or salt. By 

 comparative experiments it can be shown that the 

 flocculating power of any salt is proportional to its amount 

 up to a certain limit, when the material is so completely 

 flocculated that no further addition of salt has any 

 effect; conversely, the flocculating power of a given 

 amount of salt is inversely proportional to the quantity 



