330 EARLE C. GREGG, JR 



Concerning the destructive action of ultrasound, a general impres- 

 sion held by a few is that pressure differences still play an important 

 role. While cavitation cannot exist without large pressure differences, 

 it remains that if the pressure differences are largely responsible for the 

 destructive action there should be a great dependence on frequency — 

 that is, the sound could be "tuned" to the dimensions of the undesired 

 organism. No data on such an effect have yet been reported. On 

 the other hand, if cavitation is principally responsible for the de- 

 structive action, the elimination of this phenomenon should greatly 

 diminish the virulent action. Observations of this sort by Schmitt 

 and Uhlemeyer {12) were made on protozoa by maintaining the same 

 acoustic intensity and increasing the external pressure until cavi- 

 tation and bubble formation ceased. The destruction of the protozoa 

 was considerably lessened. Johnson {13) and Harvey and Loomis {10) 

 also made the same observation. Stanley {IJj) used a different tech- 

 nique when investigating the inactivation of tobacco mosaic virus 

 by ultrasound. He found no inactivation in the absence of the dis- 

 solved gases. In this case, cavitation was still present but due to the 

 absence of bubble formation the local agitation and temperatures 

 were not. It is important to remember that cavitation also has a 

 seeming frequency effect (very slight, however) since the cavities are 

 formed in the trough of the wave. If cavitation is responsible, this 

 may explain in part the observations of Wood and Loomis and other 

 investigators concerning the ability of smaller protozoa to "ride the 

 waves." Harvey {32) has made a rather critical investigation of 

 these biological effects and has arrived at the conclusion that most of 

 the lethal and sterilizing action of ultrasound on small organisms is 

 due to rapid fluid movements as a result of submicroscopic cavitation 

 in the presence of dissolved gases. He has also shown, particularly 

 in the case of plant cells, that cavitation and the expansion of cavi- 

 tated gases within the cells should have little effect. 



Further results of his investigations were that none of the phe- 

 nomena investigated could be connected with local heating as long 

 as the average temperature of the medium did not rise. This was 

 prevented b.y cooling the oil above the crystal and by electrically 

 shielding the medium so that radiofrequency heating (due to the 

 electrical signal applied to the crystal) did not take place. He further 

 found that the standing sound wave pattern was quite important 

 and determined to a great extent the observed phenomena — particu- 

 larly the movements and distribution of the particles in the medium. 



