86 Tks Structure of Protoplasm 



We are dealing with a thixotropic change, too, if the viscosity 

 of a concentrated sol can be reduced by shaking or stirring and if it 

 increases again when at rest. The viscosity of such a sol differs in 

 other respects from that of a Newtonian liquid. It is anomalous, i.e., 

 it does not obey Poiseuille's law for laminar flow; the amount of 

 liquid passing through a capillary in a given time depends on the 

 applied pressure in a different way. Speaking more specifically, the 

 viscous resistance is not directly proportional to the velocity gradient 

 of the laminar flow, as in Newtonian liquids. The characteristic 

 behavior of anomalously viscous, thixotropic sols and likewise of 

 thixotropic gels is correlated with the existence of a yield value. 

 This can be demonstrated in a graphic way by using a viscometer, 

 in which a ball is pulled through the viscous system in question by 

 a weight which can be increased at will; for different weights the 

 speed of the moving ball is compared.^' In a Newtonian liquid the 

 speed is proportional to the weight applied (curve 1 in Fig. 1) . In 

 a thixotropic sol or gel we have a yield value (A) : The weight must 

 exceed a certain minimum to cause the ball to move; only with 

 higher weights does its speed change in a way similar to that ob- 

 served in a normal liquid (cf. curve 2 in Fig. 1). Curve 1 refers to 

 glycerol, a typically Newtonian liquid, curve 2 to a thixotropic, 

 aqueous iron oxide gel. In other cases we may actually have a less 

 straightforward behavior: a curve which, at higher weights, is simi- 

 lar to curve 2 but which does not intersect the abscissa; instead, it 

 is curved and passes through the zero point. 



Systems showing anomalous viscosity also exhibit a characteris- 

 tic behavior with respect to their electrical conductivity. The elec- 

 trical conductivity of an aqueous solution is markedly influenced 

 by its Newtonian viscosity; as a rule, it is, in a first approximation, 

 inversely proportional to the viscosity. For instance, the electrical 

 conductivity of an aqueous salt solution having a fairly high vis- 

 cosity, owing to the presence of a suitable concentration of glycerol, 

 is lower than that of a solution of the same salt concentration in 

 pure water. The anomalous viscosity of a thixotropic sol or gel, on 

 the other hand, leaves the electrical conductivity practically un- 

 changed. The electrical conductivity of a salt solution remains the 

 same, although it may contain so much gelatin that its apparent vis- 

 cosity is about equal to that of the solution containing glycerol.^ It 

 is well known that even gels of gelatin (containing electrolytes) or 

 of soaps do not differ in their electrical conductivity from the sols 

 from which they were produced. ' Presumably the movement of the 



