Sols and Gels — Relatio7i to Protoplasmic Structure 93 



which act over smaller distances; an attraction due to electric forces 

 is more probable. The possibility has been considered that both 

 attractive and repulsive forces between the particles may be of 

 electric origin.''- But this concept is perhaps not easily reconciled 

 with the fact that the forces between the particles may be markedly 

 specific. Vanadium pentoxide, as well as benzopurpurin sols, form 

 tactoids which carry a negative electric charge. In a mixed sol of the 

 two, under proper conditions, both kinds of tactoids may be formed 



■^- i>.'**»i*.. 





*'■■..■ -J* "^ ' J 



B 



Fig. 2. 



side by side, containing only VoO-, and benzopurpurin particles 

 respectively.'^^ These two kinds of tactoids can be distinguished, 

 because those of V2O,-, are positively, those of the dyestuff negatively, 

 double refracting. The very specific behavior of some ions when 

 producing autocomplex coacervates must also be mentioned in this 

 connection. 



The nonspherical shape of the colloidal particles strongly favors 

 thixotropy. But the opinion, which has been occasionally expressed, 

 that nonspherical shape is an indispensable factor for thixotropy 

 is not correct. Thixotropic systems are known whose particles 

 hardly deviate from the spherical shape, e. g., pastes of intact starch 

 grains in organic liquids.'' 



It is becoming more and more probable''^ that the thixotropic, i. e., 

 isothermal change, sol ^^ gel, is the general phenomenon. When 

 dealing with the so-called, nonisothermal sol ^ gel transformation, 

 as we have it in aqueous sols of gelatin, agar, or methyl cellulose,^'' 

 we must distinguish between (1) the actual sol ^ gel transforma- 



