The coefficients of thermal conductivity of copper, mercury and glass are respectively ${\mathrm{K}}_{\mathrm{c}}, {\mathrm{K}}_{\mathrm{m}} \mathrm{and} {\mathrm{K}}_{\mathrm{g}} \mathrm{such} \mathrm{that} {\mathrm{K}}_{\mathrm{c}} >{\mathrm{K}}_{\mathrm{m}} > {\mathrm{K}}_{\mathrm{g}}$. If the same quantity of heat is to flow per second per unit area of each and corresponding temperature gradients are ${\mathrm{X}}_{\mathrm{c}}, {\mathrm{X}}_{\mathrm{m}} \mathrm{and} {\mathrm{X}}_{\mathrm{g}}, \mathrm{then}$