(1) The vapor pressure equations of liquid and solid, normal deuterium were determined by comparison of the vapor pressure of deuterium with that of liquid, normal hydrogen between 13.9 degrees and 20.40 degrees K. The triple and boiling points of deuterium were found to be 18.58 degrees and 23.5 degrees K, respectively. (2) The changes with time in the vapor pressures of liquid hydrogen and liquid deuterium at 20.4 degrees K, resulting from ortho-para conversions, were investigated. The rate of change of the vapor pressure of liquid deuterium resulting from its natural, self-conversion was found to be less than 1/40th of the natural rate of conversion for liquid hydrogen. (3) From the vapor pressure equations of deuterium and an equation of state, its latent heats were deduced by the use of the Clausius-Clapeyron equation. Two equations of state for deuterium were used: (a) the empirically determined equation for hydrogen, and (b) an equation deduced from an equation of state of hydrogen of a form required by the Bose-Einstein statistics by including the effect of mass in such approximation though the latter might be used with little change in results. The vapor pressure equation required by the third law of thermodynamics has been derived and by the use of the Debye theory values of the zero point energies and Debye Theta's are secured. These Theta's which are used to calculate the heat capacities of solid and liquid deuterium at constant pressure are calculated from the vapor pressure data. The difference between C-p and C-v is much greater for solid hydrogen than for solid deuterium indicating a larger coefficient of expansion in the case of hydrogen. This is to be expected because of the larger zero point energy of hydrogen. The heat capacity under saturation pressure of liquid deuterium is considerably less than that of hydrogen. A recalculation of the distillation data of Urey, Brickwedde and Murphy on the natural mixture of H-2 and HD shows that they secured approximately the enrichment to be expected from the theory and experimental conditions.
