130 SOIL PHYSICS AND MANAGEMENT 



all metals and metalloids have beeii produced in a colloidal state. 

 The simplest compounds of these, as oxides, sulfides, chlorides, 

 hydroxides, some carbonates, chromates, phosphates, sulfates, and 

 silicates, occur in this form. Among the organic substances that 

 occur as colloids are starch, dextrin, gum, rubber, glue, gelatine, 

 caseins, albumins, humus, and proteins in general. 



Properties of Colloids. The difference between colloids and 

 crystalloids in one of physics and not of chemistry. The chemical 

 composition is the same in whichever state they occur. Hence, a 

 study of colloids is largely a study of their physical properties and 

 characteristics. 



(a) Size of Particles. The upper limit of size for colloids is 

 near the limit of visibility with the ordinary high-power microscope, 

 which is not far from 0.0001 mm. With the most powerful micro- 

 scope some of the largest colloidal particles may be seen ; with the 

 ultra-microscope, particles 0.000005 mm. in diameter are about the 

 limit of visibility. Many smaller particles exist, but their presence 

 is revealed only by the properties of their suspensions. The parti- 

 cles larger than 0.0001 mm. give ordinary suspension and may some- 

 times show some properties of colloids. Those between the above 

 size and the molecule give colloidal suspensions, while the molecules 

 give true solutions. 



The smaller the particle the longer it will remain in suspension. 

 This is due to the fact that the specific gravity of the particle and 

 its adhering film of water have such a low specific gravity that it 

 varies but little from that of water (see page 35). 



(b) Brownian Movement. Very fine particles in water are con- 

 stantly in motion. This movement is not a definite progressive one, 

 but an irregular, jerky motion from one side to the other. Particles 

 as large as 0.01 mm. sometimes show a slow movement of this kind, 

 but it is best developed in the very finest particles. The movement 

 is increased by higher temperature. 



(c) Dialysis. Dialysis is the diffusion of a substance through 

 a membrane. Experiments show that colloids will not pass through 

 membranes or at best only very slowly. Separation of colloids from 

 crystalloids may be made in this way. 



From the following table it will be seen that dialysis takes place 

 about 80 times as rapidly with crystalloids as with colloids. This 

 is due to the fact that the parchment itself is a colloid. 



(d) Diffusion. Colloids diffuse very slowly and they do not 

 allow other colloids to pass into them. Crystalloids may pass into 



