Lambda-Temperatures of Solutions ofHe3inHe4below 1°K

Measurements have been made of the transition (lambda) temperatures at which solutions of ${\mathrm{He}}^3$ in ${\mathrm{He}}^4$ first show superfluidity by observation of the thermal isolation of a vessel which contained the solutions and which was cooled to low temperature by a paramagnetic salt. The incidence of superfluidity was accompanied by a high heat influx to the solution produced by a convection of film flow to the high temperature together with return flow in the vapor phase. The solution with the highest concentration, namely 89 percent ${\mathrm{He}}^3$, had a lambda-temperature of 0.38°K; solutions with smaller ${\mathrm{He}}^3$ content (concentrations down to 42 percent ${\mathrm{He}}^3$ were investigated) showed higher lambda-temperatures ranging up to 1.15°K. It was concluded that pure ${\mathrm{He}}^3$ could not be superfluid above 0.25°K and most probably is not superfluid down to 0°K. From the results conclusions have been drawn regarding the free energy of pure ${\mathrm{He}}^4$ in the liquid II state on the basis of the two-fluid model.