Example $4\mathrm{.}13$ : The space between two concentric spherical vessels is completely evacuated. The inner sphere contains air at $76K(-197C).$ The ambient temperature is $300K(27C).$ The surface of the spheres are highly polished $)=(0\mathrm{.}04.$ Find the rate of evaporation of liquid air per hour.

Diameter of inner sphere $=250mm$ Diameter of outer sphere $=350mm$

Latent heat of vaporisation of air $=200k\frac{J}{k}g$