The reflection of vapour molecules at a liquid surface

The rate of evaporation of a liquid may be calculated from the kinetic theory of gases if it be assumed that all vapour molecules which strike the surface enter the liquid and that, as a first approximation, the vapour behaves as a perfect gas. Under these circumstances, it follows from the kinetic theory of gases that m = mass of molecules leaving unit area per minute = mass of molecules striking unit area per minute from the saturated vapour = 14·63 Ps/√Ts gram/sq. cm. /min., where Ps is the saturated vapour pressure in millimetres of mercury at the surface temperature Ts° A.