An air bubble of diameter $6 \mathrm{~mm}$ rises steadily through a solution of density $1750 \mathrm{~kg} / \mathrm{m}^{3}$ at the rate of $0.35 \mathrm{~cm} / \mathrm{s}$. The co-efficient of viscosity of the solution (neglect density of air) is ___________ Pas (given, $\mathrm{g}=10 \mathrm{~ms}^{-2}$ ).