Methods and means for analyzing substances

An apparatus for analyzing a solid or quasi-solid sample surrounded by fluid, including amplitude modulated energy source means for applying radiant energy of a predetermined frequency and periodically varying magnitude to the sample sufficient to produce detectable periodic variations which are indicative of the sample characteristic in the surrounding fluid, said source means including frequency varying means for varying the predetermined frequency of the energy which is applied, pickup means in the fluid and spaced from the sample for responding to the detectable variations in the fluid and producing electrical signals corresponding in amplitude and phase to the detectable periodic variations, said pickup means including means forming a chamber having internal walls which reflect both light and sound so that direct or scattered light does not strike an internal wall which can produce a detectable periodic variation and so that the chamber is susceptible of being a part of a high Q acoustic resonant system, said sample being located within the chamber, and means for indicating the intensity of the electrical signals at the different predetermined frequencies of the energy.