The first measurements of the rate at which ultra-cold neutrons are produced within a vessel of liquid helium, reported in a recent issue of Physics Letters (66A, 469; 1978), strongly support the earlier suggestionR by Golub and Pendlebury (Physics Letters 62A, 337; 1977) that it might be possible to fill a neutron bottle with trapped neutrons to an unprecedentedly high density if superftuld 4He were used as the internal medium.