THE COLLOIDAL MATTER OF SOILS 163 



108. Preparation of colloids. — There are a number of 

 methods that may be used in the preparation of artificial 

 colloidal solutions, but the description of only one will 

 suffice in the present discussion. This is the use of a 

 semipermeable membrane. It has already been men- 

 tioned that crystalloids pass with ease through a membrane 

 such as parchment paper, while colloids do not. It is 

 reasonable to expect, then, that these materials may be 

 thus separated by proper adjustments. As a matter of 

 fact, such a procedure is employed in many cases. The 

 operation is called dialysis, and the membrane, itself a 

 colloid, is designated as the dialyzing membrane. 



For example, if a solution of ferric chloride to which 

 some ammonium carbonate has been added is placed in a 

 dialyzer with pure water on the outside, the hydrochloric 

 acid and other impurities gradually pass through the 

 membrane and a more or less pure colloidal solution of 

 ferric hydrate is left behind. The objection to this 

 method lies in its extreme slowness. Nevertheless, since 

 the cells of plants present a semipermeable membrane, 

 this method of preparation serves to explain many actions 

 that go on between soil and plant during the processes 

 of nutrition. In the soil the formation of colloidal ma- 

 terial is entirely a natural exertion of chemical and bio- 

 logical forces under such conditions that the particles 

 split off are in that state of division which has been desig- 

 nated as colloidal. 



It may be inferred that the quantity of colloidal matter 

 in an average soil is large, but as a matter of fact this is 

 not the case. The proportion of the soil in a colloidal 

 state at any one time is very small. It must be remem- 

 bered, however, that material is continually being thrown 

 out of the colloidal condition and at the same time more 



