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colour being complementary to the yellow tinge caused by iron 

 compounds in the clay. The proportion of water present in colloidal 

 ferric hydroxide is very variable; it parts with water on drying in 

 a similar manner to silica though it is much more irregular. The 

 variable proportion of water probably explains the variable colour 

 to different clays containing iron compounds. 



Various colloidal silicates have been found in small quantities in 

 some clays, though absent in many highly plastic clays. The only 

 exception to this is a possible " silicate of alumina " or more correctly 

 " alumino-siUcic acid " (or series of such acids), which appears to be an 

 essential constituent of clays and may be the origin of the colloidal 

 substance to which they are supposed to owe their value. 



Colloiclal organic matter, chiefly humus, may play an important part 

 in giving to clays their characteristic properties, but as some well- 

 known highly plastic clays are almost devoid of carbonaceous matter, 

 the latter cannot be the chief cause of their plasticity. Moreover, 

 the addition of certain organic colloids to feebly plastic clays does 

 not increase their true plasticity, though it may increase the cohesion 

 and stickiness of the particles. 



From the foregoing it may be assumed that the characteristic 

 properties of clays are not due to colloidal silica, alumina, ferric 

 hydroxide or organic matter, or to colloidal silicates of the alkalies 

 or alkaline earth metals, though when any or all of these are present 

 they may slightly increase the plasticity, or otherwise modify the 

 properties of the clay. 



To what Colloidal Matter do Clays owe their Character ? 



It has been suggested in the foregoing pages that the more 

 important properties of clays may be due to the presence of colloidal 

 matter in them. It has been shown that colloidal matter does exist 

 in clays and that many of the characteristic properties of clays are 

 equally characteristic of colloidal gels, though none of the latter 

 possess all the properties of a valuable plastic clay. The question 

 therefore arises, as to whether the so-called colloidal properties of 

 clays are merely coincidental with the composition of clays, or 

 whether plastic clays contain some substance or substances not 

 hitherto identified as a colloidal gel. 



There can be no doubt that most plastic clays contain a large 

 proportion of non-plastic and non-clayey material and may be 

 regarded as diluted clays. Some of the most highly plastic clays, 

 on the contrary, consist of such small particles that the non-clayey 

 matter cannot be satisfactorily separated. On the one hand, attempts 

 to separate an ideal " clay substance " by chemical or mechanical 

 methods have resulted in a material which is almost devoid of plasticity, 

 and on the other, attempts to show that clays are essentially colloidal 

 have not satisfactorily produced any definite colloidal substance 

 which can be regarded as clay. We are, therefore, compelled to 

 realise that many of the more important properties of clays are due 

 to the colloidal nature of the material, though clays are not wholly 



