F. LONDON1 has recently proposed a new conception of helium II, according to which this liquid can be regarded as a degenerate Bose-Einstein gas, that is, as a system in which one fraction of the substance—say, n atoms per cm.3—is distributed over the excited states in a way determined by the temperature, while the rest—n0–n atoms per cm.3—is 'condensed' in the lowest energy level. If T0 denotes the temperature of degeneracy, the ratio n/n0 is given by For an ideal Bose-Einstein gas, according to London, s = 3/2, but for the real fluid one should rather insert s = 5 in order to fit Keesom's specific heat measurements. The rapid pace of discovery that characterized early experimental work in superfluidity [see Nature 141, 243-244 (1938)] was equalled by theorists. In one of the great conceptual leaps in the history of physics, Fritz London proposed in April 1938 that the λ-transition in liquid helium was analogous to Bose-Einstein condensation, predicted by Einstein to occur in dilute gases. Just one month later, Laszlo Tisza extended London's proposal by invoking a two-fluid model for helium II, which could qualitatively explain the observed transport phenomena, including the fountain effect.