VII. On the velocity of second sound in liquid helium ii

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Abstract
Some recent experiments by Atkins and Osborne (1950) in the Mond Laboratory give values for the velocity of second sound at very low temperatures in close agreement with Landau's (1941) predictions. It is shown that such results cannot be obtained from any theory of Tisza-London type, unless it is postulated that other states besides the very lowest one contribute to ρs , the density of superfluid. Landau's (1941) method of calculating ρs can be applied to any model and does imply such a postulate. It is concluded that the experimental results only throw light on the correct method of calculating ρ s and cannot be regarded as evidence in favour of Landau's quantum hydrodynamics. On the other hand, the fact that liquid He3 a does not show superfluid properties can no longer be regarded as evidence against Landau's theory, as the predictions of the Landau and Tisza-London types of theory are identical for both the perfect Bose-Einstein and the perfect Fermi-Dirac gases.

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