336 EARLE C. GREGG, JR. 



of iodine ions into iodine molecules but depolymerization, inversion 

 of sugar, and effects on hydrolysis have also been reported. The 

 most plausible explanation to date is that most of the effects are due 

 to high local temperatures generated by dissolved gases filling the 

 small cavities. This, as was mentioned in the preceding section, was 

 substantiated by the independent experiments of Porter and Mari- 

 nesco. 



The above phenomena, however, can explain neither luminescence 

 nor the formation of hydrogen peroxide from oxygen and water, 

 which appears to be the basis of the acceleration of some chemical re- 

 actions. This has been brought out by the fact that many reactions 

 are accelerated by the cavitation of certain dissolved gases (air and 

 oxygen) while they are unaffected by others. In order to overcome 

 this difficulty, Richards (19) has proposed a balloelectrical theory. 

 This theory is based on the concept that j ust as spraying a liquid into 

 a gas produces charged drops, the spraying of holes in a liquid (cavi- 

 tation) must produce charged bubbles. The high local potentials 

 might then be the cause of the various chemical reactions. Measure- 

 ments on cavitating fluids showed large variations in potential at 

 the points of cavitation and none where the liquid was relatively un- 

 disturbed. This theory also accounts for the fogging of photographic 

 plates when exposed to ultrasonic sound. Needless to add, both the 

 high local temperatures and potentials may be present at the same 

 time in any cavitating fluid so that an analysis of any given chemical 

 or biological effect may involve one or both phenomena. 



Other physicochemical effects of ultrasonics are the alteration of 

 sensitive metastable systems such as the yellow to red change in 

 mercuric oxide at room temperature and the detonation of explosives 

 by vibrating glass surfaces in air. 



4. Emulsification and Dispersion 



One of the first things demonstrated -by intense ultrasound was the 

 transformation of two immiscible liquids into a very stable emulsion 

 (11). Water and oil, and water and mercury were first tried and 

 since then a variety of mixtures has been used. Richards (19) has 

 shown that emulsification is very strong at the boundary surfaces 

 between the liquid and vibrator and also between the liquid and walls 

 of the vessel. Other investigators have shown that the emulsions 

 produced are different depending on whether or not a gas is present 



