ELASTIC GELS 55 



enough to maintain their shape under the stress of their own weight. This 

 means that they will be molded to the shape of the vessel in which the gelation 

 has occurred, and will retain the shape of that vessel after being removed 

 from it. 



Solutes diffuse through gels almost as rapidly as through pure water, 

 unless the gels are very concentrated. The diffusion of a solute through 

 a gel is easily demonstrated by a simple experiment. A gelatin sol is allowed 

 to solidify in a test tube which is then inverted in a shallow dish containing 

 a solution of a dye such as methylene blue. Within 24 hours diffusion of 

 the dye into the gelatin gel can be detected. Because of the fact that there 

 are no convection currents in a gel to complicate the results, this is perhaps 

 the best visual method of demonstrating the diffusion of solutes. 



Two equal quantities of an electrolyte, one dispersed in water, the other 

 in a gel of equal volume, will conduct an electric current almost equally 

 well. In other words ionic mobility is apparently as great when the ions are 

 dispersed in a gel as when they are dispersed in pure water. 



The velocities of chemical reactions occurring in a gel medium are not 

 appreciably different from the velocities of the same reactions occurring under 

 the same conditions of solute concentration, temperature, etc., in a water 

 medium. Neither of the last two general statements are strictly true for 

 very concentrated gels. 



The above three properties of gels distinguish them clearly from the solid 

 or amorphous states of matter. These properties must be reconciled with any 

 acceptable theories of the structure of gels. 



Elastic Gels. — Two general types of gels are usually recognized, the elas- 

 tic type, and the non-elastic type. The best known example of the latter is 

 the silica gel. Elastic gels are the important type biologically. Gelatin- 

 water and agar-water gels are probably the best-known examples of this type 

 of colloidal system. When such a gel dries a gradual and consistent shrink- 

 age in its volume ensues until desiccation is complete. After desiccation the 

 dry matter of the gel will imbibe water, but no other liquid. Gels in which 

 the liquid phase was other than water will imbibe, after desiccation, only the 

 liquid originally present. 



Elastic gels are generally heat reversible. When heated such gels are 

 converted into sols isolation) and when cooled such sols resume their gel 

 condition {gelatW7i). Usually this reversal of state can occur a number of 

 times to a colloidal system without greatly affecting its physical properties 

 when in either the sol or gel condition. The temperatures at which the sola- 

 tion and gelation of a given colloidal system occur are not identical. The 

 processes differ in this respect from the melting and freezing of a solid. For 



