Properties of these gases are discussed with particular reference to their applicability as dielectrics. Physical properties — boiling point (dew point), freezing point, critical temperature, critical pressure, specific heat, thermal conductivity, viscosity, density — and natural convective heat transfer characteristics are discussed. Data on dielectric strength and corona inception voltages as determined in various field configurations and at various gas pressures are presented. Results of tests on the thermal stability of these gases in the presence of various materials of construction used in electrical apparatus are reported.