Graham's law of effusion 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 whererate1 is the rate of effusion for the first gas, rate2 is the rate of effusion for the second gas, M1 is the molar mass of gas 1, M2 is the molar mass of gas 2.