Compression of SolidHe3andHe4to 20 000 Bars

For purposes of comparison, we have measured the compression of both solid ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ at 4.2 °K up to 20 000 bars pressure. The relative volume changes of the solid samples were measured in a direct piston displacement apparatus. It was found that both isotopes fit a Murnaghan finite strain relation. From this compressibilities were computed. Within experimental error, the two isotopes have the same compressibility throughout the range. As might be expected from its larger zero-point energy, ${\mathrm{He}}^3$ has the slightly greater molar volume at all pressures.