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A general method is presented for the evaluation of multipole-multipole force constants, and the temperature dependence is calculated by representing each H3 triangle as a superposition of a ring of charge qr and a triangle of charge qt, with qt increasing and qr decreasing as the temperature falls through Tc. Changes in these multipole forces with decreasing temperature depress the initial gradients of the transverse acoustic modes by coupling with the librational doublet, with the latter exhibiting soft-mode behaviour. The calculations also predict that the stability of the structure is very dependent on the magnitude of the interaction between clusters, in agreement with theory, and that the chloride structure is more stable and has a higher transition temperature than the iodide, both of which are in agreement with experiment.

The properties of an assembly of n + 1 reacting isomers in ideal solution is examined with reference to the thermodynamic theory of relaxation. It is shown explicitly how the equilibrium properties of the system can be related to the chemical reaction coefficients and to what extent the formalism of statistical mechanics can be used in this context. The matrix of relaxation times is obtained in terms of the `transition probabilities' together with their derivatives with respect to concentration. The latter terms give rise to a condition (additional to the well-known condition for microscopic reversibility) which must be satisfied if the Onsager reciprocity relations are to be valid. There is no obvious physical interpretation for this extra condition.

The authors describe the development of the physics syllabus particularly with respect to the relationship between physics and chemistry.

Attention is drawn to the importance of recognizing the various relaxation times associated with a single relaxation mechanism. The physical origin of the differences between the relaxation times is indicated and the relationships between them are illustrated by examples. A general formula is derived which relates the relaxation times for a single mechanism under different thermodynamic constraints. Connection is established between chemical rate constants and relaxation times. It is concluded that there is no intrinsic relaxation time derivable on a kinetic basis without reference to the thermodynamic conditions under which the relaxation takes place.

A Green function method is used to calculate the one-phonon infra-red absorption by uncharged point defects in a face-centred cubic Bravais host lattice. A nearest-neighbour central-force model of the host lattice is used. Changes in defect-host force constants are allowed for, but no correction is made for the relaxation of the host lattice around the defect. Choosing the distortion dipole moment parameters so as to fit the experimental results of Jones and Woodfine for xenon defects in argon indicates that a substantial distortion dipole moment results from non-radial displacements of the neighbouring atoms relative to the defect. The poor agreement with experiment obtained in previous shell-model calculations seems to be due to a neglect of this non-radial effect.

Measurements of ultrasonic relaxation in liquids and gases provide experimental data from which it is desired to obtain theoretically useful parameters such as heats of reaction and transition probabilities. It is shown here that even if full experimental results are available concerning the variation of the ultrasonic relaxation strength r with pressure and temperature it is still necessary to make at least three assumptions before thermodynamic reaction parameters can be evaluated. The main purpose of the paper is to set out a logical theory which makes the nature of these assumptions explicit. It is first shown how the experimental results can best be analysed to yield the relaxation strength. The formula connecting the relaxation strength with the reaction parameters leads to a partial differential equation which is integrated in a form convenient for practical application. The general implications of the theoretical work are discussed in relation to experimental observations and the need for further experimental work at high pressures is emphasized.

Formulae giving the optical absorption due to second-order terms in the crystal electric moment expansion are obtained. These formulae are expressed as convolution integrals of matrix outer products of harmonic lattice Green functions. The simplifications following from selection-rule requirements follow naturally from the results; the formulae are especially suitable for defect absorption calculations. The integrated absorption in both the high- and low-temperature limits is obtained and expressed in terms of the electric moment coefficients and the force constant matrix.

The author discusses the definition of the Avogadro constant and the implicit ideas about quantities and units associated with it. It is suggested that, in addition to the Avogadro constant, one should recognize the Avogadro number, N0=6.02*1023. In some ways N0 is a more natural and convenient quantity than L, although of course the two are intimately connected: L=N0 mole-1.

A list of books is given which are recommended for introductory reading in other subjects and for topical (and therefore, usually, outside syllabus) themes in physics.

K A Muller and A Rigamonti (eds) Amsterdam: North Holland 1976 pp xxii + 879 price $115.50 The central theme of this fine collection of papers is the contribution of 'nondiffraction' techniques to the study of phase transitions: nuclear magnetic resonance, paramagnetic resonance and birefringence. Most of the emphasis is – very naturally – placed on magnetic and ferroelectric transitions, but there are some articles on liquid crystals and one after-dinner speech on cooperative phenomena in biology.

L Kubat and J Zeman (eds) Amsterdam: Elsevier, Prague: Academia 1975 pp 260 price Dfl.55 This is a collection of 20 essays, notable for their unusually wide intellectual and geographical spread. Intellectually they range over physics, philosophy, biology and even economics; geographically they are drawn from the United States, Western Europe, Eastern Europe and the Soviet Union.

Robert S Shankland (ed) London: University of Chicago Press 1974 pp xxlx + 777 price £13.75 It has always irritated me (and no doubt others) that whereas third and fourth rate writers and politicians often get full biographical and bibliographical recognition, many first rate scientists have been neglected. Some redress is afforded by this handsome, subsidized publication of Arthur Holly Compton's papers on his major contributions to x ray and quantum physics, covering the period 1913–35.

Dieter Wolf 1979 Clarendon Press: Oxford University Press xviii + 462 pp price £1.5 This book is an authoritative and clear exposition of the theory of nuclear spin relaxation. What makes it specially valuable is the fact that it encapsulates within a single, unified viewpoint the major theories associated with (i) Kubo and Tomita; (ii) Bloch, Wangsness and Redfield; (iii) Hebel and Slichter.

Polarisation switching in a twin stripe laser is exploited to produce logic operations with possible applications in optical data processing and computing. Experimental results are presented showing polarisation based logical inversion, and this operation is used to enable the configuration of the laser device as an optical “clock” and optical pseudo-random pulse generator. Phase locking of the optical clock is also demonstrated.

Richard Whitley (ed) London: Routledge and Kegan Paul 1974 pp ix + 286 price £5.95 During the last 10 or 15 years sociologically oriented writers such as Roe, Kuhn, Ziman, Ravetz and Ben–David have given physicists much instruction and pleasure and some pain. Little of these responses will be elicited by the present volume.

Richard H Lyon London: MIT Press 1976 pp 388 price £13.60 This book is a missionary tract directed to sound and vibration engineers. The basic ideas are (to some extent) familiar to physicists: if you have an oscillator, or a set of 'similar' oscillators (which may be quasicontinuous in energy, or just moderately dense), then the system can couple linearly to other such systems, and one can discuss the transfer and distribution of energy.

R. M. Mazo Oxford: Pergamon Press. 1967. Pp. xiii + 166. Price £3. Not long ago I expressed the hope that someone would soon produce an intelligible account of modern theories of transport in simple fluids at a reasonable price. This has now been done—at least if you interpret 'reasonable' in the Pergamon sense—in a brilliant and satisfying little book by Robert Mazo.