THE COLLOIDAL MATTER OF SOILS 157 



influences to a slight degree. Secondly, colloids do not 

 pass readily through semipermeable membranes, as 

 parchment paper, while crystalloids do. This serves as 

 a very easy way of separating colloidal and crystalloidal 

 material. As a matter of fact, the membrane is itself a 

 colloid. Thirdly, heat and the addition of electrolytes 

 will serve to coagulate or precipitate certain colloids, a 

 property which again serves to distinguish them sharply 

 from a true solution. Fourthly, colloidal material has great 

 adsorptive power, not only for water, but also for materials 

 in solution, a quality of extreme importance in soil studies. 

 It has been shown that a colloid is a material in a 

 certain state of division, in which it exhibits properties 

 not possessed by an ordinary suspension or by a true 

 solution. It is therefore proper to speak of matter so di- 

 vided as being in the colloidal state, or colloidal condition. 

 It is not to be inferred, because the colloidal phase is 

 contrasted with the crystalloidal, that colloids are amor- 

 phous. They may or may not be in such a condition. 

 Moreover, the same material may exist without chemical 

 change either in the colloidal or non-colloidal state. For 

 example, silicic acid, ferric hydrate, gold, carbon black, 

 and other materials, may or may not be colloidal, ac- 

 cording 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 illustrations of the colloidal state w r ill not be amiss. 

 The following materials, which may exist as colloids, may 

 be for convenience grouped under two general heads, 

 organic and inorganic : 



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

 humus, carbon black, tannic acid, etc. 



