170 THE PHYSICS OF VIRUSES 



sures go, these are not large and will not normally he expected 

 to have any great effect on a virus. The data of Johnson, Baylor, 

 and Frazer (1948) quoted in Chapter 4 show that for tobacco 

 mosaic virus the rate of inactivation at room temperature and 

 10 atmospheres pressure is not appreciable. It therefore seems 

 likely that the pressure aspect of ultrasonics is not responsible 

 for their effect on viruses. 



Cavitation 



Gas nuclei in a liquid can grow with great rapidity if pressure 

 reduction occurs as in a sound wave. They also collapse with 

 great rapidity, and this growth and collapse is called car?*/a/iO?7 . 

 If no dissolved gas is present in the liquid, cavitation cannot 

 occur. 



Temperatures associated with cavitation are not known ac- 

 curately. By immersing ])owdered exjilosives which are not wet 

 by liquids and then applying sonic energy, it is possible to 

 measure the critical detonation energy. By relating this to the 

 flash temperature of the ex])losive, the local temperature can be 

 estimated. In a sound field of 20 watts/cm- at 1 inc, values up to 

 230° C were obtained. 



These factors, and their biological action, are discussed by 

 Harvey, Barnes, McElroy, Whiteley, Pease, and Cooper (1944). 



The three possible physical agencies to watch in ultrasonic 

 work are, therefore, temperature, cavitation, and pressure variation. 



Sonic Irradiation Procedures 



High-intensity sound in the upper audible range (10,000 

 cycles, or so) can be obtained with a magnetostriction generator. 

 A metal which is magnetized changes length, and if a ra]ndly 

 alternating current is fed from an oscillator into a coil of wire 

 around a rod of steel, the alternations cause change in the 

 length of the rod and these, in turn, can cause pressure changes 

 in a liquid i)laced above the end of the rod. Since the permeability 

 of the steel is high there is effectively an am])lification, so that 

 quite high powers can be fed into the liquid. Such sonic irradia- 

 tion devices are available commerciallv. The design of an 



