36 COLLOIDAL SYSTEMS 



be poured more or less readily from one vessel to another. To the unaided 

 eye they often appear to be true solutions, but examination by means of an 

 ultramicroscope reveals their colloidal nature. 



Many sols "set," forming solid, but more or less elastic systems, generally 

 called gels. Gelatin desserts, custards, and ordinary household jellies are 

 familiar examples of gels. The change of a sol to gel is called gelation. The 

 reverse change, as for example when gelatin is "melted" by the application of 

 heat, is called solation. Some authorities use the term gel in a generic sense 

 to include systems of the type just described, which they call jellies, and 

 another type of colloidal system which they term coagula. The white of a 

 hard-boiled egg is a typical coagulum. 



Sols may be classified into lyophobic and lyophilic types. In the latter an 

 appreciable affinity exists between the particles of the disperse phase and the 

 dispersion medium ; in the former no such affinity is present. When the 

 dispersion medium is water the corresponding terms hydrophobic (Gr. : 

 "water-fearing") and hydrophilic (Gr. : "water-loving") are employed. This 

 last pair of terms will be used consistently in the following discussion, since 

 from the biological standpoint colloidal systems in which water is the disper- 

 sion medium are by far the most important. The affinity between the two 

 phases of a hydrophilic sol manifests itself by hydration of the micelles. Hy- 

 dration is the association of one or more molecules of water with an ion, 

 molecule, or micelle. Hydration of ions and molecules has already been dis- 

 cussed. Hydration of micelles is in principle the same. 



Most colloidal systems composed of metallic substances dispersed in water 

 are examples of hydrophobic sols. Gelatin, agar, starch, and gum acacia sols 

 are familiar examples of hydrophilic systems. Protein sols also belong in this 

 group. Actually all possible gradations exist from highly hydrophilic sols 

 to highly hj'drophobic sols. 



Suspensions. — The general nature of suspensions has already been in- 

 dicated. Suspensions are not generally regarded as playing a very significant 

 role in living organisms. The particles of suspension size which frequently 

 can be observed in the protoplasm are apparently composed entirely of rela- 

 tively inert materials. Particles of suspension size are very common in soils, 

 and as such are an important part of the environment of the roots of plants. 

 A consideration of suspensions is also of importance in developing the con- 

 ception of the structural and dynamic aspects of colloidal systems proper. 



Emulsions. — Emulsions are systems in which one liquid is dispersed 

 throughout another with which it is virtually immiscible, the particles of the 

 dispersed liquid exceeding about O.i ^ in diameter. While other types of 

 emulsions are theoretically possible all such systems encountered in common 



