96 The Structure of Protoplasm 



in a certain range of pressures and it may, therefore, be distinguished 

 from other actions of the waves not caused by cavitation. In vacuo 

 and at a suitably high pressure, thixotropic gels are not liquefied 

 by ultrasonics, which proves that the phenomenon is produced by 

 a collapse of cavities. ^^ 



The thixotropic decrease of structural viscosity in concentrated 

 colloidal solutions can also be produced by ultrasonics as an effect 

 of cavitation. This was shown for sols of gelatin where the effect 

 was partially reversible."'- In many cases effects are observed which 

 are due to cavitation, but are irreversible and which, therefore, must 

 be explained somewhat differently. In the sols of methyl cellulose 

 mentioned above, the structural viscosity left over, after cooling a 

 gel which had been formed at higher temperatures, can be made to 

 disappear by ultrasonics.^" In colloidal solutions of hemocyanin 

 {Helix pomatia) , ultrasonics split the molecules in halves; on pro- 

 longed action, a certain percentage of eighths appears. ^^ Perhaps such 

 a splitting effect occurs to a smaller or larger extent in many solu- 

 tions of highly polymerized organic substances.^- 



Besides the destructive action due to cavitation, ultrasonics also 

 exert a coagulating effect which becomes conspicuous, both at high 

 energies and at such low ones that no more effects of cavitation are 

 observed.^"' Coagulation is very striking when ultrasonics act upon 

 fogs or smokes.^*^ It is also shown by the fact that, on emulsifying two 

 liquids like water and benzene in each other, a limiting value of 

 particle concentration is reached, where the dispersing effect is 

 balanced by the coagulation.^"^ In emulsions and suspensions of 

 suitable concentration, stationary ultrasonic waves cause an accumu- 

 lation of the particles in the nodes or antinodes, depending on whether 

 the particles are lighter or heavier than the surrounding medium.^^ 

 Coagulation may be observed where this accumulation takes place. 

 Coagulation by ultrasonics may be due to several reasons which are 

 not easily distinguished from each other. Two may be mentioned: 

 (1) Attractive forces of a hydrodynamic nature are produced 

 between particles, if the particles are suspended in a medium exposed 

 to vibration.'' (2) The so-called orthokinetic coagulation may cause 

 collisions and hence a clustering of particles. ^"^ This latter kind of 

 coagulation occurs if the particles do not have equal sizes and hence 

 move with different rates, when exposed to external forces. 



The marked effect of ultrasonics on some rheopectic suspensions 

 is due to coagulation. If a suspension of a kaolin ("Stockalite") in 

 an aqueous solution of NaCl was liquefied by shaking and exposed 



