Graham's law of effusion (also called Graham's law of diffusion) was formulated by Scottish physical chemist Thomas Graham in 1848. Graham found experimentally that the rate of effusion of a gas is inversely proportional to the square root of the molar mass of its particles. This formula is stated as: , where: Rate1 is the rate of effusion for the first gas. (volume or number of moles per unit time).Rate2 is the rate of effusion for the second gas.M1 is the molar mass of gas 1M2 is the molar mass of gas 2.