Sols and Gels — Relation to Protoplasmic Structure 95 



for a and b respectively. The rheopectic effect was produced by 

 grasping the tube containing the sol between two fingers and oscil- 

 lating it like a pendulum about the point where it was grasped. The 

 times of rheopectic setting were about 10 seconds and 40 seconds 

 for a and b respectively. In rheopexy, the nonspherical shape seems 

 to be a decisive factor in causing the phenomenon. The gel (or 

 paste) produced by rheopectic action does not appear to have the 

 same structure as the one that has set spontaneously. The difference 

 between dilatancy and rheopexy is obvious: In dilatancy the final 

 state is liquid; the system behaves as a sohd only as long as the 

 external force is acting. In rheopexy the final state is solid; the 

 external force, causing the slight movement, only increases the rate 

 of solidification. 



The mechanical forces causing liquefaction in thixotropy and the 

 increased rate of solidification in rheopexy may be replaced by the 

 action of ultrasonic waves.-^^' ^^ These phenomena deserve to be 

 discussed here, because the action of ultrasonics upon organisms has 

 already produced some interesting results and may lead to a better 

 discrimination of the manifold influence of mechanical forces upon 

 living systems.^^'* 



Thixotropic gels like those of iron oxide, etc., are liquefied by 

 ultrasonics of sufficiently high energy.^^ Experiments of this kind 

 are done by simply dipping the test tube containing the gel into the 

 oil fountain that is formed above the vibrating quartz plate gener- 

 ating the ultrasonic waves; the plate is lying in an oil bath. This 

 effect is one of so-called cavitation.^" The waves produce periodic 

 dilations and compressions in the systems through which they pass. 

 The dilations may be intense enough to cause the liquid to tear, i. e., 

 a cavity is formed, filled with the vapor of the liquid. This cavity 

 may collapse, if it again gets into a region of higher pressure. This 

 collapse of a cavity — to which we are referring if we speak of an 

 effect of cavitation — may lead to very high local concentrations of 

 energy and hence to effects like those of an explosion. It is appar- 

 ently mainly this phenomenon that produces the strongly destruc- 

 tive effects of ultrasonics.^" Cavitation is, for instance, the cause 

 of the strong dispersing action of ultrasonics upon mixtures of 

 organic liquids, such as benzene and water, where it has been inves- 

 tigated thoroughly. A collapse of cavities does not occur in vacuo, 

 the cavities formed simply increase in size; under a sufficiently high 

 outside pressure, on the other hand, cavities are not formed at all.^" 

 Hence, an action of ultrasonics due to cavitation is observed only 



