The molecular volumes at saturation of liquid normal hydrogen and parahydrogen between 14° and 20.4°K were m easured with a fused-quartz dilatometer, the amount of hydrogen being determined from the press ure of the gas in a calibrated flask at a measured temperature. The results are represented by the equations: V(n-H z) cm 3 mole-' = 24.747 -0.08005T+ 0.012716Tz V(p-Hz) em3 mole-' = 24.902-0.0888T+0.013104Tz At the normal boiling point of n-Hz (20.38°K), V(n-H2) = 28 .397 ± 0.010 and L1 V(p-n

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... ) = 0.138± 0.010. The expansivity of p-H2 is only slightly greater than that of n-H2• The change observed in the molecular volumes of Hz in passing from the state in which the m olecules rotate (O-H2) to the state in which they do no t rotate (p-H z) is opposite in direction to the change observed in other substances when passing between these two states of molecular rotation and nonrotation. The changes in "latti ce" energies are also of opposite sign. It is shown that the random state of orientation of the axes of P-H2 molecules makes the state of nonrotating P-H2 fundamentally like that of rotation in other substances at high temperatures . CONTENTS Page In liquid parahydrogen, P-H2' below 20 o IC, there is no molecular rotation since practically all the molecules are in the state whose rotational quantum number, j, is 0, whereas in liquid orthohydrogen, o-H2, (j = 1) there is "free" molecular rotation, which is demonstrated by the fact that the difference in energies of the rotational levels is the same in the liquid and gaseous phases.' The variation with 1