Several investigators have shown recently that if the changes in the internal energy of a gas take place so slowly that they cannot follow the adiabatic changes which result when a sound wave passes through the gas, sound dispersion and anomalous absorption of observable magnitudes occur. Experimentally, it is found that the absorption in the air, for example, not only depends upon temperature and humidity in a characteristic manner, unsuspected by classical theory, but at frequencies above 1000 cycles is so great as to require consideration in such technical problems as architectural acoustics and sound signaling. The findings are in good agreement with the new theory. The presence of small impurities (less than one percent) in gases may change the speed of sound at certain frequencies as much as several percent: and the absorption coefficient as much as several hundred percent. Absorption measurements for frequencies between 1000 and 40,000 cycles inO2containing small impurities ofH2, He,H2O, H2S, NH3, CO,CO2, O3, C2H2, or one of many other hydrocarbons throw light on the nature of molecular collisions. A new device is thus available for studies in molecular physics, and acoustics has become a potent ally of the quantum theory.