& Boyd (1971). Further inspection of Fig. 2 suggests that the structure initially induced by the transition at 4800 kg cm -2 is not stable within the time scale of the X-ray exposure. For example the spacing of the (110) plane of the tetragonal structure at 4800 kg cm -2 still shows a little expansion with a further increase in pressure of 100 kg cm -2, as compared with Fig. 1, where steady reduction of the (110) spacing is seen above 5000 kg cm -2. In all these pressure regions, the (011) and (002) planes show steady decrease in spacing with increasing pressure and are more compressible than the (110) plane. Thus the crystal of adamantane after the transition is fairly anisotropic in the strains induced by hydrostatic pressures, being more compressible in the c direction than in the directions perpendicular to it. The strains are, however, isotropic before the transition, as shown by the cubic structure sustained under pressure (Fig. 1). The unit-cell dimension of the cubic structure just before the pressure transition is a= 9.20 A (at 4600 kg cm -2) and the calculated volumetric strain of the crystal at this stage is 6"9 %.It is worth while to note that at p, and 20°C, the crystal transition in adamantane arises in such a way that one of the original cubic axes shrinks 4.2 % and becomes parallel to the tetragonal c axis, while both of the other two axes expand 0.5% in the tetragonal [110] and [110] directions (numerical values were calculated at two pressures, 4600 and 5000 kg cm-2). The net effect is an abrapt volumetric contraction of 3-1%, which corresponds to abrupt molecular rearrangements toward denser packing.