129 



allied work of other observers until 1896, when Rohland* investigated 

 tlie subject further, and found indications that the colloidal nature 

 of clay appeared likely to explain many of the facts noted in regard 

 to plasticity. 



The nature of the colloid material apparently existmg in many 

 clays has already been described. In attempting to explain plasticity 

 as being due to these colloids, it is assumed that some or all of the 

 pores of the clay are filled with a colloidal solution (gel) obtained 

 by the partial hydrolysis of the clay, and that the larger the 

 proportion of pores so filled, the fatter and more plastic will be 

 the clay, provided the proper ratio of granular material to colloid 

 gel is retained. 



Rohland^ and others have further shown that the addition of 

 trifling amounts of electrolytes often produces great changes in the 

 lalasticity of a clay, and suggest that this characteristic of colloids is 

 a strong argument in favour of the connection between the colloidal 

 material in clay and plasticity. All electrolytes (such as acids) which 

 jdeld hydrogen-ions on dissociation, increase the plasticity of clay, 

 whilst those (such as alkaUes) which yield hydroxyl-ions make a 

 clay more fluid. 



Plasticity is not, however, entirely due to the presence of colloidal 

 matter in clays, though the effect of colloids in increasing plasticity 

 cannot be denied. Hermann and others maintain that the presence 

 of inorganic colloids in clay has never been conclusively proved. It 

 should be noted that clay may be suspended in water and then 

 precipitated or deflocculated indefinitely without impairing its 

 plasticity. This is not usually the case with true mineral colloids, 

 which usually set irreversibly and do not return to the colloidal condi- 

 tion. Moreover, the whole of any individual clay grain is not softened 

 upon the addition of water. Repeated wetting and pugging does 

 not materially alter the size of the grains or change their general 

 outline or appearance. This would not be the case if the clay were 

 softened and reduced to a homogeneous mass, wetted, and subse- 

 quently broken up with the formation of new grains when it was 

 dried and ground. Whether wet or dry, under the microscope, the 

 grains retain the appearance of a sharply-defined body. 



Another difificulty has been pointed out by J. M. van Bemmelen, 

 viz., the rapidity with which colloids lose their power of absorbing 

 water. This suggests that clays of great geological age cannot contain 

 active colloids produced when the clay was formed, though they may 

 contain colloidal substances derived from adventitious materials — 

 organic or otherwise — at a comparatively recent period. The fact 

 that many highly plastic clays appear to be free from such extraneous 

 colloids only increases the difficulty regarding the latter as the cause 

 of plasticity. Other objections of equal or greater weight may be 

 urged against any single theory yet published on the causes of 

 plasticity so that much further work requires to be done. 



Summarising the results of the numerous theories and experiments 

 made, plasticity may be said to be due not to one, but to several 

 causes, the chief of which are : — 



(i) The nature of the molecules of " true clay " present. 



X 11454 I 



