134 PROTOPLASM 



Emulsions possess many extraordinary peculiarities. Among 

 them is the effect of resting on the type of the emulsion. One- 

 half of a sample left on the laboratory table, while the other half 

 is being used, may reverse to the opposite type on shaking 

 fifteen minutes later, owing simply to changes going on while 

 resting. Remarkable also is the effect of rest on the quickness 

 with which emulsions are formed. Intermittent shaking pro- 

 duces an emulsion much more rapidly than does constant shaking. 

 When forming, emulsions are especially sensitive. The method 

 of preparation has a pronounced effect on the success of the 

 emulsification and also on the type. Any housewife of a genera- 

 tion ago will testify to this on the basis of her attempts to make 

 mayonnaise. The mayonnaise emulsion problem is commercially 

 now well in hand, but the reasons for the success are little 

 understood. 



No system, except the living substance itself, exhibits more 

 idiosyncrasies than do emulsions, and for most of these we have 

 no adequate explanation. 



Supersonic Waves. — A novel way of forming emulsions has 

 arisen as a result of studies by the French physicist Langevin 

 (1923) on sound waves of exceedingly short length — so short as 

 to be below audibility by the human ear. They are known as 

 ultraphonic, ultrasonic, or supersonic waves. They are produced 

 by the vibration of a quartz crystal (plate), brought about by 

 subjecting the crystal to an alternating current of high frequency, 

 the wires of which are wrapped about the crystal. The crystal 

 trembles, owing to the electrical oscillations to which it is sub- 

 jected. This trembling brings about the production of waves in 

 the medium in which the quartz is suspended ; the medium usually 

 employed is a neutral oil. The vibrations are purely mechanical, 

 like those of a tuning fork but much shorter. The range in 

 audibility of sound waves for the human ear is in air from 200 m. 

 to 16 mm. Supersonic waves are all below 16 mm., generally 

 2 or 3 mm. Variations in the frequency of the current cause the 

 production of waves of different length. 



If a coarse mixture of oil and water is subjected to supersonic 

 vibrations, a very fine emulsion is produced in short time. If a 

 gel that exhibits thixotropic qualities (page 150) is set in vibration 

 by supersonic waves, it will collapse just as it does when mechani- 

 cally agitated by more crude disturbances. 



