X. ULTRASONIC VIBRATIONS 313 



/ = PV2py = 0.34 X 10^ ergs/sec./cm.2 = 0.34 watt/cm.^ 



When liquids are degassed, their natural cohesive pressure becomes ef- 

 fective and they will stand a negative acoustic pressure. It has been 

 found that under these circumstances the total negative pressure re- 

 quired to cause cavitation is equal to the sum of the cohesive pressure 

 (tensile strength) and the ambient pressure. The cohesive pressure 

 appears to be a variable quantity and depends to quite an extent 

 on the previous history of the liquid. Once a liquid has cavitated, 

 it will cavitate at a lower acoustic pressure. It will also require some 

 time to return to its previous state. 



It has been found that the optimum pressure for pronounced cavi- 

 tation in water is approximately two atmospheres. This corresponds 

 to an intensity of 1.35 watts/cm. ^ Briggs, Johnson, and Mason (22) 

 have attained higher intensities than this without cavitation by puls- 

 ing or driving the source for very short time intervals. There is 

 seemingly some time delay in producing cavitation. Briggs, Johnson, 

 and Mason have formulated a theory of this time delay based on 

 Eyring's theory of viscosity, plasticity, and diffusion, which agrees 

 quite well with experiment. Harvey (32) and collaborators have 

 treated in a similar manner the formation of gas bubbles in blood and 

 other liquids. Among other interesting phenomena, Harvey has 

 found that subjecting liquids to a high hydrostatic pressure (1000 

 atmospheres) prior to investigation results in a condition in which only 

 very severe blows will cause bubbles to form even when the container 

 is exhausted to the vapor pressure of water. See also Novotny (36) 

 and Pease and Blinks (57) . 



For heavy viscous liquids, the power required for cavitation is 

 approximately two to four times that required for light liquids. This 

 is explained by the fact that viscous liquids have a high cohesive 

 pressure. A linear relationship apparently exists between the sound 

 cavitation amplitude required and the viscosity of the medium. As 

 far as degassing is concerned, to obtain 1 cc. of air per second from 

 water saturated with air, Sorensen found that it required 51.2 kilo- 

 watts at 194 kilocycles, 72.6 kilowatts at 380 kilocycles, and 87.4 

 kilowatts at 530 kilocycles, a rather inefficient means of degassing a 

 liquid. 



From all the above figures, it is seen that, in order to produce 

 cavitation and the ensuing biological actions, relatively large amounts 

 of ultrasonic power are required. Considering the fact that the loud- 

 speaker in an average radio radiates about 10 ~^ watts per square 



