There are, however, at least two other sources of 

 loss experienced by a bubble: diffusion of gas and evapora- 

 tion of water vapor into and out of the bubble during the 

 acoustic cycle. It was not possible to find any investiga- 

 tion of these losses in the literature, presumably because 

 the main interest in the past has been centered on bubbles 

 close to resonant size. Moreover, the resonant bubbles 

 are adequately described by their viscous, thermal, and 

 radiation losses. However, for a given total volume of 

 gas phase within a sample, the surface area available for 

 diffusion and evaporation is inversely proportional to the 

 radii of the bubble. Accordingly one should expect a 

 greater effect due to diffusion and evaporation for the 

 nuclei. The theory of the growth of an air bubble in a 

 sound field by diffusion processes has been investigated 

 in a recent paper 45 and provides a possible approach to 

 the problem of determining energy losses. Reference may 

 also be made to an experimental paper concerned with 

 ultrasonic cavitation traceable to the growth of air nuclei. 46 



Particulate Matter as a Source of Nuclei 



The greatest part of the particulate matter in the 

 samples was undoubtedly of organic origin. The question 

 therefore arises whether organic material can produce 

 bubble nuclei and also act as the stabilizing center for a 

 nucleus. 



The evidence found in the literature indicates that air 

 bubbles are almost never found in living cells. 47 Bubbles 

 may form, however, if a cell is injured by pinching or 

 cutting and then exposed to reduced pressure. Bubbles 

 may form inside a cell even though the cell wall is not 

 broken by being pinched and they may form in a different 

 place than the pinched spot. This is explained by the 

 unavoidable twisting strains set up in the cell material 

 when manipulated. 47 



There is still another possibility that not only will 

 explain the above experiments, but also the observations 



93 



