This paper deals with the forces between polar molecules and their relation to gas imperfection. In addition to the interaction of permanent dipoles, other directional intermolecular forces, such as dipole-induced dipole interaction, quadrupole forces and the effect of molecular shape may be important. These are discussed quantitatively and intermolecular potentials proposed to allow for them. Detailed expressions for the corresponding second virial coefficients are obtained from statistical mechanics and applied to experimental data for some polar gases.

The effects of molecular collisions on the hyperpolarizabilities of spherical atoms are considered. Closedform expressions are given for the second hyperpolarizability of a pair of atoms in the presence of dipolar coupling for five cases, namely, a static electric field, third-harmonic generation, electric-field-induced secondharmonic generation, the Kerr effect, and four-wave mixing. The longest range contribution to the mean hyperpolarizability varies as R-6; it is less than one-third of the magnitude of the effect deduced by Donley and Shelton, but agrees with that obtained by Hunt. For a dissimilar pair of atoms, a collision-induced first hyperpolarizability proportional to the inverse fourth power of the separation is predicted.

The generalnature ofmolecular interactions is discussed; a molecule is defined as an atom or group of atoms whose binding energy is much larger than the thermal energy kT. The interaction energy is broken down into electrostatic, induction, dispersion, resonance and overlap energies. The theory of the longrange interaction energy is developed using quantum mechanical perturbation methods, and the electrostatic, induction and dispersion energies written in terms of the free-molecule electric moments and polarizabilities. The theory of short-range overlap forces is briefly considered. Various manifestations of intermolecular forces are discussed, including equilibrium properties of fluids, the structure of crystals and large molecules, spectroscopic properties, molecular bearn scattering, chemical effects and forces between macroscopic bodies.

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This paper sets out to define the underclass and then test the predictions of three competing theories in the underclass debate. Using the National Child Development Study for the analysis it is found that an 'underclass' suffering from a lack of qualifications, low cognitive ability and chronic joblessness exists. The validity of making a distinction between the working class and an 'underclass' has often been questioned both because of the dubious history of such a distinction and because it is not believed that such a distinction is empirically true. The results in this paper contradict this assertion by finding the underclass to be distinctive from the working class in terms of patterns of family formation, work commitment and political allegiance. The distinct attitudes of the underclass, when coupled with evidence of inter- and intra-generational stability of membership, provide early evidence that a new social class, the underclass, may now exist in Britain.