λAnomaly in the Heat Capacity of Solid Hydrogen at Small Molar Volumes

Apparatus for the determination of properties of solid hydrogen at volumes between 15 and 23 cc/mole is described. The results of measurements on the $\lambda$ anomaly in the heat capacity of solid hydrogen containing large concentrations of orthohydrogen are reported for molar volumes from 15.8 to 22.6 cc. The $\lambda$ temperature was found to vary with intermolecular separation $R$ as $R^{-5\mathrm{}}$, with a small contribution from a higher exponent. This is in agreement with the expected relative contributions of electric quadrupole and valence interactions to the intermolecular potential. The relative contribution of the $\lambda$ anomaly to the total anomalous entropy was estimated. The $\lambda$ anomaly was found to have at least two distinct peaks.