X. ULTRASONIC VIBRATIONS 333 



and chemical investigations in which the nodes of standing waves 

 would produce no action. Stumpf, Green, and Smith (SI) found that 

 the shape of the container affected the rate of bacterial disintegra- 

 tion, a direct exhibition of this phenomenon, and it is felt that this 

 maj^ also explain to a great extent the earlier observations that small 

 protozoa could "ride the waves" w^hile the larger ones could not, 

 the "waves" being the nodes and antinodes of a stationary sound 

 field pattern. 



Instead of employing mechanically moving reflectors, which 

 might be cumbersome to use in any given investigation, it is possible 

 to frequency modulate the sound source directly by standard elec- 

 tronic circuits {34)- These should prove much more adaptable par- 

 ticularly at the higher frequencies. Offhand, it appears that the pos- 

 sibility of frequency modulating the sound source should provide a 

 very powerful tool in a more complete investigation of biological and 

 chemical action of ultrasonics in small containers, where the effects of 

 the boundaries become important. The influence of the boundaries 

 on the resultant sound field pattern and the consequent effects can- 

 not be emphasized too strongly. 



2. Thermal Effects 



When dealing with investigations of this sort, it is important not 

 to overlook the heating effects of the sound. There are in general 

 two main effects: high local temperatures due to cavitation in the 

 presence of dissolved gases and a general increase in temperature due 

 to the absorption of the sound energy by the medium. 



Porter and Young (16) have reported the conversion of an acid 

 azide into an isocyanate, which is a known case of readjustment of 

 atoms within a molecule by vibrational energy or heat. A thermom- 

 eter immersed in the solution showed a change of 0.3°C. during the 

 run while a temperature of 90° was necessary to decompose the azide 

 at the rate measured. It is a plausible assumption that the neces- 

 sary temperatures were local phenomena generated by cavitation. 

 These local heats would not appreciably raise the temperature of the 

 over-all liquid but might increase tremendously the reaction rate at 

 the points where the local temperatures were high. Marinesco (17) 

 further proved this by detonating such labile materials as NI3 with 

 ultrasound. No explosions occurred when the liquids were degassed. 



Many organic materials show a large general rise in temperature 

 due primarily to the absorption of the radiation and not to the effect 



