On the basis of the theory of capillarity, the process of formation of a wetting perimeter when a spherical particle touches a planar liquid surface is analysed taking into account the line energy of this perimeter. The minimum size of particle which can float is calculated and an accurate estimate is made for the minimum time of touching of particle and surface. The kinetic energy of collision between an air bubble and the particles is used to calculate the maximum size of particles which can remain attached for flotation. Flotation data suggest that a lower limit for the size of isolated particles which can be floated is of the order of 1 pm and a comparison of this radius with the theoretical limit gives a value of dyn for the line energy of the wetting perimeter. Predictions of the

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The thermodynamics and mechanics of three-phase contact line systems are developed. According to Gibbs, the line tension of the contact line is determined as an excess quantity which brings into correspondence two three-phase contact systems: a real system with a transition region and an idealized system without it. Estimates of this quantity are given for several cases including that of soap films, where long-range interaction forces play an essential role. The line tension can be negative or positive, and this is discussed in detail. From the potential formulas for the work of nuclei formation in supersaturation systems are obtained. For heterogeneous nucleation, a negative line tension implies condensation without a barrier, and experimental data on the critical supersaturation for nucleation of water on hexadecane are interpreted on that basis. These data are obtained by using the reverse Wilson chamber (RWC), which is described. The value of line tension thus obtained is -1.9 X 10"5 dyn. This value is close to the values of the line tension measured by other experimental techniques, which are also described. Measurements on Newton black film systems show that the line tension can change sign as the salt concentration is varied.