Discovered by Thomas Graham [32K GIF] of Scotland in the 1830s (I think!).
Consider samples of two different gases at the same Kelvin temperature.
Since temperature is proportional to the kinetic energy of the gas molecules, the kinetic energy (KE) of the two gas samples is also the same.
In equation form, we can write: KE1 = KE2 Since KE = (1/2) mv2, (m = mass and v = velocity) we can write the following equation:
m1v12 = m2v22
Note that the value of one-half cancels.
The equation above can be rearranged algebraically into the following: the square root of (m1 / m2) = v2 / v1
You may wish to assure yourself of the correctness of this rearrangement.
This last equation is the modern way of stating Graham's law.