The critical behaviour of ethylene and hydrogen

Optical techniques have been used to study the behaviour of ethylene and hydrogen near their liquid-vapour critical points. From measurements of the coexistence curve of ethylene over the reduced temperature range 1.5 x 10⁻⁶ < t < 4.5 x 10⁻², where t — (Tc — T)/Tc and Tc is the critical temperature, we find the critical exponent β = 0.327±.002 and the corrections-to-scaling exponent ∆ = 0.46±.02. Similar measurements for hydrogen over the range 3.2 x 10⁻⁵ < t < 7.0 x 10⁻² give β = 0.326 ± .002 and ∆ = 0.46 ± .02. Measurements of the compressibility of hydrogen give the critical exponent [Formula Omitted] = 1.19 ± .05 and the critical amplitude ratio [Formula Omitted] = 5.2 ± .4. With the exception of ∆, which is slightly lower than its predicted value of 0.5, the results for these universal quantities are in agreement with theoretical predictions. The leading coexistence curve amplitude for hydrogen, B₀ = 1.19±.03, is lower than the corresponding values for ethylene, B₀ = 1.56 ± .03, and for other room-temperature fluids. This decrease is in qualitative agreement with the predictions of a theory of quantum effects on critical behaviour. Measurements of the coexistence curve diameter for both fluids show an anomaly near the critical point having a form consistent with the predicted t¹⁻α temperature dependence. These results are in agreement with a recent theory of the effects of many-body forces on the diameter; the hydrogen data indicate that these forces are attractive in that fluid. This suggests that quantum mechanical exchange interactions are important near the critical point of hydrogen.Science, Faculty ofPhysics and Astronomy, Department ofGraduat