The calculation of viscosity of a degenerate Bose-Einstein gas is in progress. A preliminary estimation shows that the atoms belonging to the lowest energy state do not take part in the dissipation of momentum. Thus, the viscosity of the system is entirely due to the atoms in excited states. This simple description allows us to account in a qualitative way for the well-known transport phenomena in helium II, though in the more exact treatment certain modifications have to be introduced, which are to be discussed in a detailed publication.

(1) Measurements of the damping of an oscillatingv cylinder will show the effect of atoms in excited states only. To a first approximation the viscosity, measured in this way, should be the same as the viscosity∗ of helium gas at the same temperature. The value of the viscosity coefficient of helium gas at T ~ T0 is \L ^ 10"5 C.G.S. units6, while measure ments in helium II just below the X-point2 give jz = 3 x 10~5. Parallel measurements of the viscosity of helium II and helium gas in the corresponding range of temperature would provide us with an indication as to what extent the simple picture adopted here has to be refined.

(2)It is, however, not the viscosity coefficient [JL which determines the flow velocity of helium II moving under the nfluence of a pressure gradient through a capillary. In this case, the fraction of substance consisting of atoms in the lowest energy state will perform-like a 'superfluid' liquid of viscosity (JL ^ 0-some sort of turbulent motion the flow velocity of which does not depend on the bore of the capillary. In addition, the atoms in excited states, behaving like a gas of pressure /^ nkT, will diffuse in a similar way to, say, molecules in a solution under the influence of a gradient of osmotic pressure. Thus, the total flow represents a rather complex combination of both these effects. It will largely depend on the ratio (nQ - n)/n and, therefore, on T, in agreement with experiment3. According to this interpretation, a temperature gradient should arise during the flow of helium II through a thin capillary.

(3) The so-called fountain phenomenon4 of helium II is an inverse process to (2). If one maintains a temperature difference between the ends of a capillary, a gradient of density of excited atoms, n, and, thus, of pressure is produced. In consequence, (a) the excited atoms will diffuse towards the colder end, and (6) the super-fluid fraction of the liquid moves in the opposite direction. In the case of a wide tube, these currents must be equal and no resulting flow will be observed. If, however, the capillary is sufficiently narrow, the rate of the process (a) becomes reduced and the temperature gradient causes a surplus convection current opposite to heat flow. This picture can account for the great values of the heating current required to maintain a temperature difference at the ends of the capillary5. Simultaneous measurements of the heating current and the total convection of substance could provide us with information about the relative magnitude of the processes (a) and (6).

A detailed discussion of the problem will be given in the Journal de Physique. I am greatly indebted to Dr. F. London for the opportunity of seeing his paper before publication.