Bubble Nuclei 



Pure water without any air bubbles has a very high 

 tensile strength. 36 This is in stark contradiction to the 

 behavior of natural fresh or salt water. This discrepancy 

 is usually explained by the presence of gas nuclei in any 

 naturally occurring water mass. The above equation 

 determining the radius of an equilibrium bubble can be 

 considered as the criterion distinguishing bubbles and 

 nuclei. Bubbles smaller than the critical size must some- 

 how be stabilized, or they would soon go into solution. 

 The only agent capable of doing so seems to be particulate 

 matter such as dust. * Filtering or centrifuging are 

 capable of removing the nuclei. This fact can be considered 

 as substantiation of the above viewpoint. Gas nuclei can be 

 removed by applying high pressures to the sample, thereby 

 forcing the small air masses into solution. 



The dividing line between nuclei and bubbles will ob- 

 viously change with the hydrostatic pressure. Nuclei close 

 to the critical size may grow into bubbles, if p is reduced. 

 This is precisely what happens when the sample is pumped 

 up into the cavity. 



The very high attenuations measured shortly after 

 sampling, or after the samples were stirred, can perhaps 

 be explained by the development of bubbles which were too 

 small to be seen by direct visual observation. The attenua- 

 tion remaining after the sample was left undisturbed and 

 also the attenuation measured for nonsaturated water is 

 difficult to explain as an effect of bubbles. It may be 

 remarked in passing that the method of wiping the cavity 

 walls seemed to be quite efficient in removing at least the 

 larger nuclei, since bubbles rarely developed on the sides 

 after a thorough wiping, even though the water was strongly 

 supersaturated. 



Ref. 36, p. 95-97 



91 



