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A Monte Carlo method has been used to study a simple cubic Ising ferromagnet in a random quenched magnetic field. The Hamiltonian for this model is ℋ==JΣ(ij)σiσj−ΣiHiσi, where σi,σj=±1, J is the nearest-neighbor interaction constant, and the field Hi=tH is fixed at each site with ti=±1 at random and Σti=0. L×L×L lattices with periodic boundary conditions have been studied for a range of H and T. As expected we find a ferromagnetic ordered state which for small H undergoes a second order phase change to the paramagnetic state with increasing temperature. A finite size scaling analysis of the preliminary data suggests that the critical exponent β is substantially smaller (β∼0.2) than the usual 3-dim Ising value of 0.31. Results obtained for small lattices indicate that below kT/J-2 the transition becomes first order suggesting that a tricritical point appears on the critcal field curve.

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The coexistence curve and critical opalescence of a binary liquid mixture, triethylaminewater, have been reinvestigated. The measurements showed that the coexistence curve appears to follow a cubic relation t o within about 0.003" of the critical solution temperature, and in this range, there is no evidence of a flat region. Furthermore, within the range of the values of s/X (where s = 2 sin (8/2), X = wavelength in the medium, and s/X varied from 1.2 X lo-' to 7.0 X kl) and temperatures studied, the light scattering data are in good agreement with the Debye theory (J. Chem. Phys. 31, 680 (1959)). T h e molecular interaction range of Debye was found to be 5.1 f 0.3 A.The large density fluctuations of any system near its critical point are accompanied by local fluctuations in refractive index. Such a system, being optically inhomogeneous, scatters light very strongly and therefore it looks turbid. This is the so-called "critical opalescence." An equation for light scattering froin a critical binary liquid mixture can be formulated as follows (1): where I(8) is the intensity of light scattered a t a n angle 8 (between secondary and primary ray) a t a large distance r froin a scattering volume V, Id and I, are scattered intensities due t o density and concentration fluctuations, respectively, I 0 is the primary intensity, X is the wavelength of the light in the medium, k is the Boltzillann constant, T is the absolute temperature, fl is the compressibility, p is the density, n is the index of refraction, c is the concentration of the second component, P is the osmotic pressure, 42 is the volume fraction of the second component, and the symbol H represents a factor (2) of the following form.where l i j is the respective range of inolecular forces of component 1, component 2, and the mixture. The expressions Wij/oioj represent the respective cohesive energy densities. Equation [I] is a classical scattering formula corrected for the effect of the energy connected with the existence of local gradients of concentration fluctuations. I t is an approxiination which breaks down if we extend our n~easurenlents over large ranges of s/X by ineans of small angle X-ray scattering (3, 4, 5 ) . Furthermore, very long range

A specially designed light-scattering photometer with a measurable angular range of 15°–140° and with a temperature control of better than 0.001°C was used to study the angular dependence of the scattered intensity from binary critical liquid mixtures, consisting of a homologous series of alkanes as one of the two components, and β,β′-dichlorodiethyl ether (or chlorex) as the other component. Both components were purified by preparative gas chromatography and subsequent vacuum distillation. Special care was taken to prevent contamination of the systems from moisture. Scattering data were obtained by means of cylindrical light-scattering cells of different diameters (6 and 8 mm i.d.), detectors of different slit-widths (one set consisting of two slits: 1 × 10 mm each and 20 mm apart, and the other set: 0.5 × 10 mm each and 20 mm apart) and incident light of different wavelengths (λ0 = 365, 436, and 578 mμ). Phase diagrams for n-decane–chlorex and n-dodecane–chlorex were carefully determined in order to properly locate their critical mixing points. From our studies we have observed that (1) the classical theory of Ornstein–Zernicke and Debye provides a very good representation of experimental facts in the intermediate-temperature distances over large ranges of s / λ; (2) deviations seem to exist at small temperature distances (say ΔT < 0.03°C) for the n-dodecane–chlorex system. Estimates on the exponent η show qualitative agreement with Fisher's theoretical prediction of 0.059; and (3) the reciprocal scattered intensity extrapolated to zero angle has the form lim K → 0 Ic− 1 = f[T − Tc]γ, where f and γ are constants with respect to temperature. We estimate the true value of γ to be between 1.1 and 1.3 for our systems.