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The heat of fusion of solid parahydrogen has been measured for the first time at pressures above that of the triple point. The measurements were carried out up to a pressure of 338 atm. The heat of fusion was found to be a linear function of the pressure, and its values are well reproduced by the equation ΔHf = 0.04415 P+28.04, in which ΔHf is the heat of fusion in calories per gram mole and P the pressure in atmospheres. The value given by this equation for the triple point of hydrogen is 28.04, which compares with literature values of 28.03, 28.08, and 28.3 cal/mole. A few independent measurements were also made of temperatures corresponding to various pressures along the melting line for parahydrogen; most of the measured temperatures agree within 0.1°K of those previously reported in the literature.

The molar volume (density) of solid oxygen in equilibrium with vaporThe molar volume (VB) of solid parahydrogen along the melting line up to a temperature of about 24°K and a pressure of about 400 atm has been determined b~ two indepen~ent met~ods: direct meas.urement and computation from the heat of fusion and change m volume durmg meltmg. Results obtamed by the two methods agree with each other within about ±0.2%; they are both referred to b~low as "experimental." The empirical linear equation V.=27.1788-0.283044 T reproduces the experImental results with a mean deviation of about 0.15%.

Effect of free volume and temperature on the structural relaxation in polymethylphenylsiloxane: A positron lifetime and pressure-volume-temperature study Experimental data are presented for the molar volume of solid parahydrogen at five different temperatures over the range 11.49° to 19.97°K and at pressures from 0 to 350 atm. In making the density measurements, it was found that there is a fairly well-defined region of temperature and pressure in which solid hydrogen is viscoelastic. At higher pressures the solid is rather hard and rigid.A diagram is included which gives an idea of the PVT relations for solid parahydrogen up to a pressure of about 400 atm. The accuracy with which the points on this diagram are known ranges from ±0.2% along the melting line to ±5% at points far from the melting line. A striking feature is the shape of the isobars in the neighborhood of the melting line; at pressures above about 150 atm, each isobar passes through a maximum at a molar volume which is greater than the corresponding molar volume along the melting line. It appears that the thermal coefficient of expansion of solid parahydrogen is negative in a narrow range of temperature and pressure.

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