X. ULTRASONIC VIBRATIONS 337 



as a third phase. Emulsions containing gas are not only much easier 

 to produce, but also much more stable. 



In a similar manner, low melting metals and alloys have been dis- 

 persed in liquids. The particle size averages about 0.5 /x. Another 

 aspect of this effect of ultrasonics is the decrease in grain size of 

 metals with treatment of the crystallizing melt. This is to be ex- 

 pected with all crystallizing substances since the ultrasonic agitation 

 breaks off small particles of the growing crystals, which in turn act as 

 nuclei. This dispersive effect is also being used in preparing certain 

 pharmaceutical compounds. For example, Chambers and associates 

 (W) have been able to disperse sulfathiazole into such fine particles 

 that, when suspended in a water or a salt solution, a cream is produced 

 that can be injected through fine hypodermic needles. The emul- 

 sions produced by ultrasonics are often much more stable than those 

 produced by colloid mills or similar devices. There is a series of tech- 

 nical products appearing commercially, ranging from cosmetics to 

 photographic plates, that can be produced with higher quality by 

 ultrasonic emulsification than any other way. 



5. Coagulation Effects 



While ultrasonics have a strong dispersive effect on emulsions 

 and liquid sols, they have an opposite effect on aerosols, that is, they 

 exhibit a strong coagulating action on liquid or sohd particles sus- 

 pended in a gas. This coagulating action has received impetus lately 

 in attempts to precipitate fog, smoke, and industrial waste. Since a 

 new medium is involved, large generators designed primarily for air 

 operation have been developed. Allen and Rudnick (25) have re- 

 ported an air siren capable of delivering a power exceeding 2 kilo- 

 watts over a frequency range of 3 to 19 kilocycles. With the sound 

 energy from such a siren, they have been able to ignite cotton wads in 

 six seconds and steel wool in sixty seconds. In the case of the cotton, 

 only the first inch was burned, showing that the acoustic energy was 

 almost totally absorbed in that thickness. It was also possible to 

 heat a beaker of water from room temperature to boiling in about 

 seven minutes. Paper and cloth were also shredded in this sound 

 field. When the siren was adjusted to produce a vertical standing 

 wave field, small objects such as marbles (^ inch in diameter) and 

 small coins floated at the velocity antinode surfaces. 



The sound field also produced a burning sensation in the hand when 



