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In this letter we address the long-standing controversy which originated from differences between the experimental and theoretical values of the scaling exponent (n) for the critical amplitude ξ0 of the correlation length in polymer solutions. Using small-angle neutron scattering we examine the dependence of ξ0 on polymer molecular weight in polystyrene-methylcyclohexane-d14 solutions. We find that the experimental values of ncr = 0.195 ± 0.010 in the critical region of phase demixing as well as nmf = 0.255 ± 0.009 in the mean-field domain around the Θ temperature agree with the predictions of scaling theory (ncr = 0.185, nmf = 0.25).

Critical opalescence in mixtures of Bi-Zn, Pb-Ga and Bi-Ga has been observed in the scattering of 53 A neutrons. Separation of quasi-elastic from inelastic scattering has been used to separate the long-range concentration fluctuation part of the cross section from other effects. To a first approximation, this cross section agrees with the Ornstein-Zernike theory of critical scattering.

The temperature dependence of co-operative modes of motion in liquid lead has been observed in the range 336-725 ?c, using the inelastic scattering of cold neutrons. The positions and widths of modes near a frequency of 12?5 ? 1012 rad s-1 do not change significantly with temperature. However, the width of the non-propagating mode at Q = 2?2 ?-1 increases with temperature in a manner suggestive of single-particle motion. Simple models are employed to discuss these processes, although further theoretical work is needed to explain the data fully.

For pt. I, see abstr. A48149 of 1968. A study of the temperature dependence of critical opalescence in the Bi-Ga system has been made for solutions of five different concentrations, by means of the inelastic scattering of slow neutrons. At each concentration the spectra of neutrons scattered from the solution are separable into a Q-dependent quasi-elastic region and an inelastic region largely independent of Q. The former are associated with scattering from the concentration fluctuations and the latter with the high frequency cooperative modes of the system. The inelastic regions of the spectra are largely independent of temperature and the concentration of the solution, whereas the elastic regions are sensitive to both these parameters. The critical indices eta and delta are evaluated.

A simple heating arrangement for use in neutron scattering experiments up to temperatures of 900°C is described as are its application to studies of liquid metal samples.