1573 
It is felt that bulk cavitation is not simply another direct 
manifestation of the same property of sea water as that controlling 
the formation of a spray dome, but depends on something like this 
property in an entirely different way. The nucleation of a new phase 
in the bulk of a metastable medium has been treated statistically’. 
It is found that over a small range of increasing static tensions 
the rate of formation of stable nuclei increases enormously. This 
effect makes the notion of a unique breaking tension a useful one at 
least under static conditions, but in the rapidly varying pressure 
field of a shock wave it may be that more detailed account must be 
taken of the nucleation process. Photographs of cavitation (Ref, 18, 20 
21) suggest that cavitation bubbles continue to grow for some distance 
behind the front of a negative wave and can become quite large and 
oceur in such numbers as to occupy a considerable volume fraction 
when the cavitation is severe. One hopeful feature seen in photo-= 
graphs of bulk cavitation is the considerable uniformity of bubble 
size in regions which presumably have had the same pressure history. 
At the other extreme, when the pressures encountered are relatively 
small, as in our measurements, cavitation can still seriously modify 
a negative wave even though the cavitation would appear only as a 
slightly increased turbidity of sea water. 
16.5 In order to illuminate some of the patent obscurities that 
still remain connected with bulk cavitation, it would be most useful, 
as a start, to make photographic observations of the intensity and 
