By Karl F. Herzfeld

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If one assumes that the probability of energy transfer per collision (Z _1 ) is the same in the liquid and in the gas, these data can be used to calculate the time between collisions in the liquid and can be compared with those given by various theories of the liquid state, as discussed in Sees. 83-86. If the assumption just presented is right, it seems curious that the absorption coefficient increases with the temperature, since Z decreases. 16 INTRODUCTION To investigate this, one must have a considerable number of precise thermodynamic data about the liquid.

A new problem arises, that of separation of shear and compressional effects, both of which are present in longitudinal sound waves. This has not been difficult up to now, since the relaxation times of the relaxation effects discussed so far — transfer of vibrational energy — are much longer than the relaxation time for shear viscosity. Therefore, in the frequency region important for relaxation of internal energy transfer, the coefficient of viscosity could be treated as constant and the "classical absorption*' subtracted (a possible exception is the transfer of rotational energy in gases).

An absorption 100 times too high. , high energy) collisions, as in gases. To test this conclusion, the following procedure is adopted in Sec. 95. As the main relaxation time in most Kneser liquids has not been measured directly, one calculates a relaxation time which is necessary to give the measured low frequency absorption. This is only true if the extra absorption is due to slow energy exchange between vibrational and external degrees of freedom, and if the whole vibrational energy (know from spectroscopic or thermal data) has one dominant relaxation time.

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