88 The Structure of Protoplasm 



bentonite,*^ of myosin,^ and tobacco mosaic virus/" of dibenzoyl- 

 cystine^^ and of barium malonate^- in a medium of water and alcohol. 

 Aqueous solutions of gelatin have been found to show thixotropy, 

 both as concentrated solutions and as gels.^^ 



Coarse suspensions containing particles with a diameter of 1^ 

 and more may also be thixotropic: They behave as a liquid while 

 being shaken and settle to a solid paste when shaking stops. This 

 behavior is again found very frequently with aqueous suspensions 

 of clays, slates (Solnhofen slate) , and many powdered minerals 

 (mica, iron oxide, jet, etc.)^^; with finely powdered mercaptoben- 

 zothiazol in many organic liquids (benzine, carbon tetrachloride, 

 chloroform, benzene, toluene, etc.)^''; and in suspensions of many 

 pigments in oils.^'' Bentonite is a particalrly good example of a sub- 

 stance forming thixotropic systems both in colloidal and in coarse 

 suspensions.'^ The mechanical properties of coarse suspensions are 

 important because they enable us to understand better the mecha- 

 nism of these phenomena. Hence I shall have to refer to them fre- 

 quently, although protoplasm is certainly a truly colloidal system 

 containing very much smaller particles. 



The other limiting case is that of dilatancy. So far, it has been 

 investigated only in coarse suspensions. Although observed and 

 named by Osborne Reynolds^ ^ in 1885, it has only recently been 

 recognized as a remarkable counterpart to thixotropy. Osborne 

 Reynolds used the term when describing the behavior of moist 

 quartz sand: It whitens and appears to be dry when the foot falls 

 on it and becomes wet again when the foot is raised. An aqueous 

 suspension of finely ground quartz powder (particle size 1 to 5 |.i) 

 at a concentration of about 44 per cent of the solid is strongly 

 dilatant^: A glass rod can easily be moved through the mass at 

 low speed, but an enormous resistance is set up if the speed is 

 increased above a certain limit. Using the viscometer mentioned 

 above, curve 3 (cf. Fig. 1) is observed: The suspension behaves like 

 a Newtonian liquid at low speeds but, from a certain higher speed, 

 the horizontal part of the curve implies a solid behavior. In this 

 part of the curve there are some intricacies which are better dis- 

 cussed later. Suspensions of intact starch grains in water are 

 strongly dilatant too." 



There is every reason to believe that colloidal solutions may also 

 be dilatant. A colloidal solution of silicic acid of a suitable con- 

 centration and pH is found to be extremely viscous but Newtonian 

 in its behavior; it becomes hard and brittle, breaking up to a white, 



