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ECOLOGICAL separation of the African grazing ungulates has been found to be difficult1, although some authors have observed that these ungulates often take part in grazing successions in which species follow each other in characteristic sequences during their seasonal movements2. The migratory populations of the Serengeti take part in such a succession; first zebra (Equus burchelli), second wildebeest (Connochaetes taurinus albojubatus) and lastly Thomson's gazelle (Gazella thomsoni). The semi-migratory topi (Damaliscus korrigum) tends to associate with zebra. The work described here was designed to test the hypothesis that the species in such a grazing succession make use of different levels of the herb layer and might therefore be expected to take different proportions of the plant parts available.

THIS article is the first report on an extensive programme of work on the structure and spectra of glasses, in particular of vitreous silica. The principal aim of the project is to compute the properties of models based on plausible atomic arrangements and realistic interatomic forces. It is hoped that by comparing the calculated properties of these models with experimental data from real glasses, we can successively modify the models until optimum agreement between theory and experiment is obtained. This process should lead to an improved understanding of the structure and atomic vibrations of the glasses considered.

H. K. LIVINGSTON1has recently attempted to relate the Brunauer – Emmett – Teller and Harkins – Jura equations by means of the Gibbs adsorption isotherm for solid surfaces. However, he has only applied his method to one of the limiting conditions of the Brunauer – Emmett – Teller equation. We have extended this method to include the complete form of the equation, in order to direct attention to the great importance of the value of constant n in these relations.

ONE method of studying the thermodynamic properties of simple liquids is to consider the packing statistics of finite model assemblages of rigid spheres1. This finite model approach yields not the well known pair distribution function, which describes the average distribution of atoms about a typical atom in an infinite liquid, but rather a pair distribution histogram R(ρ). For an assemblage containing N spheres, NR(ρ)δρ gives the number of pairs of spheres the centres of which are separated by distances in the interval (ρ, ρ + δρ). Of course, in the limit of an infinitely large model (N→∞) and infinitesimally small histogram interval (δρ→0), R(ρ) becomes equivalent to the usual pair distribution function. One of the difficulties associated with a finite model is that edge effects seriously impair the usefulness of R(ρ) as an approximation to the radial distribution function. Clearly, any method of eliminating the edge effects from R(ρ) is of considerable interest.

IT is easily shown that for any type of internal structure the power-frequency spectrum sg(ƒ) of the current fluctuations appearing at the control grid of a triode valve is related to the spectrum sa(ƒ) of the anode current fluctuations by where Yg(ƒ) is the increase, from the ‘cut off’ to the working conditions, of input admittance at the grid with the anode effectively earthed; gmis the high-frequency mutual conductance of the triode. In the case of the ‘induced’ (transit-time) grid noise, and for moderate angles, the relevant component of grid admittance is largely susceptive, being where Ctis the value of the ‘space-charge capacitance’ measured on the valve. Thus, setting The modifications to (2) introduced by distribution of electron velocities and trajectories are discussed elsewhere1, and direct experimental verification is also described.

IN some respects, the atoms in simple liquids can be regarded theoretically as behaving like almost rigid bodies. Thus several authors1,2have been able to shed light on basic properties of the liquid state simply by analysing the packing statistics of large two and three-dimensional model aggregates of hard spheres. One very important characteristic of such an aggregate is the pair distribution histogram R(ρ); for a model containing N spheres, N R (ρ) δ ρ gives the number of pairs of spheres the centres of which are separated by distances in the interval (ρ,ρ + δ ρ). In the limit of an infinitely large model (N→∞) and infinitesimally small histogram interval (δρ→0), R(ρ) becomes formally equivalent to the familiar pair distribution function, which describes the average distribution of spheres about a typical sphere in the infinite aggregate. In this limit, R(ρ) can be taken to represent the pair distribution function for atoms in an unbounded liquid, and may thus be used in calculating thermodynamic properties of the liquid state. In practice, however, one must deal with finite models and here—quite apart from the more obvious limitations imposed by finiteness—edge effects can seriously restrict the validity of the histogram R(ρ) as an approximation to the pair distribution function of a real liquid.

WHEN bicrystals are subjected to a constant shear stress parallel to the boundary plane, the grain boundary sliding velocity decreases with time1–4. We wish to report some experiments which demonstrate the important part played by crystal slip in determining this characteristic of boundary sliding. Where slip impinges on the grain boundary, irregularities are produced which prove to be responsible for the progressive reduction in boundary sliding velocity.