130 NATURE AND PROPERTIES OF SOILS 



and other materials, may or may not be colloidal, according 

 to circumstances. The fineness of division is the explanation 

 of colloidal properties. In order to place such a discussion on 

 a more understandable basis, a few additional illustrations 

 may not be amiss. The following materials, which may exist 

 in a colloidal condition, are for convenience grouped under 

 two general heads, organic and inorganic : 



Organic: Gelatin, agar, caramel, albumin, starch jelly, 

 humus, some bacteria, carbon black, and tannic acid. 



Inorganic: Gold, silver, hydrated ferric oxide, arsenious 

 sulphide, zinc oxide, silver iodide, Prussian blue, and the like. 



72. The properties of colloidal materials. — In general, 

 there are certain properties which materials in a colloidal 

 state exhibit and by which they are distinguished from true 

 solutions. In the first place, since they are not in true solu- 

 tion, they exert little or no effect on the freezing point and 

 the vapor pressure of liquids. Some colloids have absolutely 

 no effect on these properties, while others, as they allow a 

 certain small amount of true solution to take place, do possess 

 such influences to a slight degree. Secondly, colloids do not 

 pass readily through semi-permeable membranes, such as 

 parchment paper or pig's bladder. Their diffusive powers 

 are low. This serves as an easy way of separating colloidal 

 and non-colloidal material. Thirdly, heat and the addition 

 of electrolytes will serve to coagulate certain colloids, a prop- 

 erty which again serves to distinguish them sharply from a 

 true solution. Fourthly, colloidal material has great ab- 

 sorptive power, not only for water, but also for gases and 

 materials in solution, a quality of extreme importance in soil 

 phenomena. 



Many colloids are coagulated by the addition of an elec- 

 trolyte, 1 the phenomenon often being spoken of as floccula- 



1 An electrolyte is any substance which has the ability when in solution 

 to carry an electric current, the substance suffering decomposition there- 

 by. The current is carried by the liberated ions. Hydrochloric acid, 

 for example, dissociates into ionic hydrogen and ionic chlorine, the 



