Second Virial Coefficients ofHe3-He4Mixtures between 2 and 4°K

$P-V$ isotherms of a gaseous mixture containing 54.75 atomic percent ${\mathrm{He}}^3$, 45.25% ${\mathrm{He}}^4$ have been measured at 2.146 and 3.944°K. The values of the second virial coefficients, $B_{\mathrm{mix}}$, obtained from these isotherms are, respectively, -144.5 and -70.0 cc/mole, as compared with -144.9 and -72.8 cc/mole computed from the experimentally determined second virial coefficients of the pure components and the theoretical solution nonideality parameter. It is shown that the solutions are very nearly ideal and that in the temperature range 2 to 4°K reliable values of $B_{\mathrm{mix}}$ can be obtained for all compositions.