VISCOSITY 219 



tion all indicate either high consistency or basic structural prop- 

 erties such as are characteristic of gels. It may be that those 

 structural features, which are typical of jelHes and responsible 

 for their elasticity, sometimes lead to the false interpretation 

 that the protoplasm is of high viscosity. For example, a mov- 

 ing particle, either a metalUc one magnetically attracted or 

 another centrifugally thrown through a jelly (gelatin or proto- 

 plasm) will be hindered in its movement by both viscosity and 

 structural features. The first may be low, and the latter wholly 

 responsible for the slow movement; thus will a high viscosity 

 value be obtained when actually only structural resistance has 

 been encountered. 



The opposing views on the viscosity of protoplasm may be 

 explained in a similar way. One method (microdissection) 

 reveals the viscosity of the mass as a whole, while another 

 (Brownian movement or centrifuging) indicates the viscosity of 

 only one phase of the system — the aqueous dispersion medium. 

 One may think of a sponge ; as a whole it is moderately firm, yet 

 particles suspended in the water which permeates it would move 

 through it with the same ease as through water, for that is what 

 they are in. A situation comparable to this exists in certain 

 gels which are quite rigid to low stresses but offer no resistance 

 to the diffusion of substances through them. Somewhat similar 

 is the behavior of starch. If one's finger is moved slowly through 

 a mixture of starch and water, the impression of a very thin 

 liquid is gained; but if the finger is pulled up suddenly, it sticks 

 as in cement. One is dealing here with a "brittle liquid" 

 which can be powdered by rubbing between the fingers but will 

 flow again as soon as left to stand a while. Slow movement of 

 the finger leads one to believe that the starch mixture is of low 

 viscosity, while rapid movement indicates high viscosity. The 

 same behavior is to be observed in wet quicksand. The fore- 

 going phenomenon is a thixotropic one (page 150), such as is 

 characteristic of a number of gels. If zinc oxide is mixed with 

 alcohol (in about equal parts) and transferred to a closed con- 

 tainer, it appears to be rigid, and the vessel may be turned upside 

 down without there being any tendency for the mixture to flow. 

 But if the vessel is shaken vigorously, the mixture splashes 

 about like water. Protoplasm also shows thixotropic reactions, 

 and its general behavior is that of gels of this character. 



