218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



A TOOL FOR RESEARCH 



A discovery of particular importance was made in 1932 at the 

 Massachusetts Institute of Technology. During the course of a lec- 

 ture, Prof. Peter Debye discussed Brillouin's theory of the dispersion 

 of light and X-rays by heat motion treated as a system of elastic waves 

 at which Bragg reflections take place. Debye predicted that the 

 periodic variations in density in a liquid traversed by ultrasonic waves 

 would give rise to the diffraction of light traversing the ultrasonic 

 field. Prof. F. W. Sears, who happened to be in the audience, im- 

 mediately thereafter set up the experiment. He immersed a quartz 

 plate with metallic electrodes in a glass trough, of rectangular cross 

 section, filled with carbon tetrachloride, and applied a radio-frequency 

 voltage to the crystal, thus sending a train of ultrasonic waves down 

 the trough. A source of monochromatic light, a slit, and a lens were 

 so arrranged that a parallel beam of light was sent through the liquid 

 perpendicular to the path of the sound waves. After passage through 

 the trough the light was gathered by another lens and, true to the 

 prediction of Debye, formed, instead of a single image of the slit, a 

 beautiful series of its diffraction images. Thus was born the Debye- 

 Sears effect. 



The ultrasonic waves in the liquid set up regions of strong compres- 

 sion and rarefaction with different indices of refraction of light. 

 These regions act like a phase grating (echelon) to produce the vari- 

 ous diffraction images. From the spacing of the images and the wave- 

 length of the light the sound wavelength can be determined, which, 

 together with the frequency of the sound, permits the determination 

 of the velocity of the sound in the liquid. The measurement of the 

 velocity of ultrasonic waves in a given medium by this method, and 

 by interferometric and pulse methods, permits the determination of 

 various molecular properties which are of interest to both the physi- 

 cist and the chemist. For example, these measurements permit the 

 determination of the adiabatic compressibility, which, in turn, per- 

 mits the computation of the specific heat at constant volume, otherwise 

 calculable only by means of complicated thermodynamic relations. 

 From such measurements the relation between the compressibility 

 and the concentration of solutions was determined, permitting the 

 test of a number of interesting questions in the modern theory of 

 electrolytes. Theory predicted that the molar compressibility of 

 electrolytes should vary as the square root of the molar concentration, 

 a prediction that was confirmed by these ultrasonic methods. The 

 measured variation of the velocity of sound with frequency, not pre- 

 dicted by classical theory, leads to a determination, via quantum statis- 

 tics, of the lifetimes of the excited vibrational states of various atoms 

 and the collision efficiency for excitation. 



