Liquid helium is stored at its boiling-point temperature of 4.2 K in a spherical container (r=0.30 m). The container is a perfect blackbody radiator. The container is surrounded by a spherical shield whose temperature is 79 K. A vacuum exists in the space between the container and the shield. The latent heat of vaporization for helium is 2.1x104 J/kg. What mass of liquid helium boils away through a venting valve in one hour?