The liquid phase of the helium isotope He had early been found to present perplexing
superfluid behaviour. This was noted, e.g., by H. Kamerlingh Onnes, who had received the Nobel Prize in physics 1913. Eventually, in 1938 F. London suggested that superfluidity could be a manifestation of bosonic condensation of the helium atoms. This suggestion was supported by the fact that no similar effect was seen in the isotope 3 He , which represents Fermi-Dirac statistical behaviour. However, for a long time it was difficult to connect this to the physical properties of the fluid. Then in the 1950s, O. Penrose and L. Onsager related superfluidity to the long-range order displayed by a highly correlated bosonic system. This allowed them to derive an estimate of the amount of condensed atoms in the liquid. They found only 8 %, which is because the strong interactions in liquid helium make it deviate significantly from the ideal noninteracting gas. The superfluid transition occurring at the temperature of 2.17 K, however, still somehow seems to be related to the condensation discovered by Einstein.