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We give a concise review of the empirical properties of liquid and solid 3He-~He mixtures and their phenomenological interpretation.The bulk of the paper is about dilute solutions of 3He in liquid 4He at temperatures well below the tricritical point, where the roton and phonon excitations are comparatively unimportant. We describe the thermodynamic properties in terms of the Landau-Pomeranchuk 3He quasiparticles and the effective interaction between them, introduced by Emery and Bardeen, Baym and Pines. The scattering amplitude, needed to fit the low temperature transport properties, and the effective interaction are related, provided the multiple virtual scattering calculated by Fu and Pethick is included. The multiple scattering should always be included, even for very small concentrations. We present the evidence for the velocity dependence of the effective interaction, and urge that this also be taken into account in the interpretation of experiments. We give a short description of spin-polarized liquid mixtures and of the possibility of pairing superfluidity in solutions of He in liquid 4He. The existence of supersaturated solutions may be a way to attain p-wave pairing at accessible temperatures. Because of phase separation, the concentration of 4He in dilute mixtures 4 3 473 Van der Boog, Husson, Disatnik and Kramers [104] Esel'son, Nosovitskaya, Pogorelov and Sobolev [105] Spin diffusion coefficient Candela, McAlister, Wei and Vermeulen [89] Gully and Mullin [93] Murdock, Mountfield and Corruccini [107] Fenner and Luszczynski [106] Mass diffusion constant Vvedenskii and Peshkov [108] and Pobel [111] Magnetic susceptibility (by NMR) Ahonen, Paalanen, Richardson and Takano [65] 3" m an. .,..I TABLE I. Continued o~ Specific heat Owers-Bradley, Main, Bowley, Batey and Church [46] Van der Zeeuw, Mudde and Van Beelen [112] Polturak and Rosenbaum [113] Greywall [114] Phase separation Nakaumra, Fuji, Shigi and Nagao [140] Osmotic pressure Van de Klundert, Bos, Van der Meij and Steffens [116] Normal density p. Sobolev, Esel'snn, Nosovitskaya and Pogorelov [67] Pogoreiov, Esel'son, Nosovitskaya and Sobolev [66] Speeds of first and second sound u t, u 2 and attenuation o/l, Rudavskii, Chagovets and Sheshin [ 118] Wiegers, Jochemsen, Kranenburg and Frossati [84] Adamenko, Rudavskii, Tsyganok and Chagovets [119] Chagovets, Rudavskii and Goncharov [121] Rudavskii and Chagovets [120] Grivor'ev, Dyumin, Dikina and Svatko [179] Rudavskii and Chagovets [103] Greywali and Paalanen [85] [91] Dyumin, Esel'son, Dikina and Kotenev [122] Van der Boog, Husson, Disatnik and Kramers [104] Greywall [124] Van der Boog, Husson and Kramers [123] de Voogt, de Haas, Weiber and Kramers [126] de Voogt and Kramers [127] Rockwell, Benjamin and Greytak [125] X = 0.44-2.6% X =0.6-0.85% X=3-5% X

We present both an improved model and new experimental data concerning the problem of melting in multilayer adsorbed films. The model treats in a mutually consistent manner all interfaces in a stratified film. This results in the prediction of substrate freezing, a phenomenon therrnodynamically analogous to surface melting. We also compare the free energies of stratified films to those of homogeneous films. This leads to an orderly classification of multilayer phase diagrams in the vicinity of the bulk triple point. The results of the model are compared with the experimentally known systems. Of these, only methane/graphite exhibits melting from homogeneous solid to homogeneous liquid in multilayer films. The systems Ne/graphite and Ar/graphite, studied by Zhu and Dash, exhibit surface melting and substrate freezing instead. We observe experimentally, by means of pulsed nuclear magnetic resonance, that melting in methane adsorbed on graphite extends below the film thickness at which the latent heat of melting is known to vanish. The multilayer melting curve in this system is a first-order prewetting transition, extending from triple-point dewetting at bulk coexistence down to a critical point where the latent heat vanishes at about four layers, and apparently extending to thinner films as a higher-order, two-dimensional phase transition. It would therefore seem that methane/graphite is an ideal system in which to study the evolution of melting from two dimensions to three dimensions.