N. M. Comber 381 



by a neutral calcium salt. lu these low concentrations the calcium 

 hydroxide is insufficient to produce a dominating precipitating action on 

 the surface colloids and the deflocculatiug action on the cores prevails. 



The concentration above which calcium hydroxide becomes the better 

 flocculant is much less with a lean clay than with a fat clay. In a lean 

 clay, where the proportion of effective emulsoid matter is low, less 

 calcium hydroxide is required to cause the maximum precipitation. But 

 because the amount of emulsoid matter is relatively small, the superior 

 calcium hydroxide flocculation is never very marked. In a fat clay the 

 relative effective amounts of emulsoid matter is high and more calcium 

 hydroxide is required to cause precipitation. Experiment shows that 

 emulsoid colloids such as silica protect the particles of some substances 

 and stabilize their suspensions. A fat clay contains a large amount of the 

 protective and stabihzing colloid and more colloid is accordingly required 

 to coagulate it, and precipitate the suspension. Also, because there is so 

 much emulsoid matter in the fat clay the abnormal effect of calcium 

 hydroxide, when it is attained, is very pronounced. 



This subject may now be considered from an entirely different view- 

 point. 



If a clay is treated with an excess of calcium hydroxide, it is floccu- 

 lated. Two or three washings, however, will reverse the flocculation (see 

 Appendix, paragraph 2), but no ordinary amount of washing with pure 

 water will deplete the soil of hme from the "lime requirement" point of 

 view. Again, therefore, it is clear that a certain easily removed excess of 

 calcium hydroxide is necessary to maintain the flocculated condition of 

 wet clay. 



Quite crudely, but in order to arrive at a more precise conception, it 

 may be assumed that when lime has been absorbed by a sour clay, there 

 is at one extreme a part of the lime, correcting the sourness, which is not 

 removable by ordinary water washing, and at the other extreme a part, 

 causing flocculation, which is easily removed by water washing. 



It might be supposed that the lime which is held irreversibly (or 

 relatively so) has reacted with the core of the particle, while the easily 

 reversed absorption is an action of the colloidal surface. That view, 

 however, is untenable, for experimental evidence has been described to 

 show that by extraction with dilute acid a soil may easily be brought to 

 a maximum base absorbing power. Acid of concentration considerably 

 below normal, will usually remove all the base which can be afterwards 

 replaced by shaking with calcium bicarbonate. Extraction of the soil 

 with more concentrated acid, which will more effectually attack the 



