CHAPTER/ IV. 



THE COLLOIDAL PROPERTIES OF SOIL. 



GRAHAM in 1861 introduced the term " colloids," to denote 

 those bodies which, like glue, tend to form jellies rather than 

 crystals, and which possess certain properties distinguishing 

 them from crystalloids, such as high power of absorbing water 

 and also dissolved substances from their solution. The idea 

 subsequently arose that colloids were a distinct group of 

 bodies, and when clay was observed to possess colloidal 

 properties chemists went to much trouble to try and isolate 

 the colloidal constituent : Schloesing (245^) described a pro- 

 cess that required several weeks for its performance. Later 

 on, however, it was recognised that colloids are not a group 

 of substances but a state into which most solid substances 

 can be brought : their properties are attributed to their highly 

 extended surface. Any substance, therefore, that can be 

 brought into a sufficiently finely divided state, or can be got 

 into a " web " structure so as to make its surface sufficiently 

 large, will show colloidal properties. 



Van Bemmelen was the first to show how completely the 

 clay and humus in the soil behave like colloids. And, as both 

 these substances dominate the soil to a considerable extent, 

 it is not remarkable that the soil as a whole possesses colloidal 

 properties. 



It is not my purpose to write a chapter on the properties 

 of colloids. The soil student must, however, make himself 

 thoroughly familiar with them l because he will find at every 



1 Convenient accounts of the properties of colloids are given by Hatschek in 

 An Introduction to the Physics and Chemistry of Colloids, London, 1913 ; by Wo. 

 Ostwald, in Colloid Chemistry, translated by Fischer, London, 1916; and by R. 

 Zsigmondy, Chemistry of Colloids, translated by E. B. Spear, New York, 1917. 

 For a good discussion see W. D. Bancroft, Journ. Phys. Chem., 1914, 18, 552 ; 

 and 1916, 20, 85. 



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