These are the expressions that still appear in contemporary textbooks on statistical thermodynamics. Referring to the particles in the ground state, he stated ‘They do not contribute, however, to the pressure, since their kinetic energy, (and momentum), is zero’— his italics. It would appear that he believed that, because these particles were in the ground state of the system, they could not possess kinetic energy. By omitting the kinetic energy of the particles in the condensate, however, London created a number of conceptual difficulties. The very first of these is the question of what happens to the kinetic energy of these particles at temperatures below TB, since, as each one drops into the ground state, it carries with it an energy ≈0.77kTB. Thus, at any temperature T < TB, the particles should make a contribution U0(T ) = 0.77N(1 − (T/TB)3/2)kTB to the internal energy. Expression (11) then becomes