That gases actually do move, and at rates depending on their specific nature, was rendered probable—apart from this explanation of Boyle’s law—by many phenomena observed by chemists and physicists in the eighteenth and early part of the nineteenth century. It was known from the observations of Leslie in 1804 that specifically light gases moved or diffused faster than heavy gases. Attempts to determine these rates were made by Schmidt in 1820, and by Graham in 1846, both of whom found that the rate of movement of a gas was independent of its chemical nature, and was determined solely by its mass: _gases move at rates inversely proportional to the square roots of their densities_. The following table given by Graham shows the experimentally ascertained relative rates for a number of gases compared with the rates demanded by the “_law of gaseous diffusion_.” Column one gives the name of the gas; column two, the observed rate of diffusion; and column three, the square root of the density of the gas (air = 1): _Gas._ _Time of diffusion._ _√density._ Air 1 1 Hydrogen 0.276 0.263 Marsh gas 0.753 0.745 Ethylene 0.987 0.985 Nitrogen 0.986 0.986 Oxygen 1.053 1.051 Carbon dioxide 1.203 1.237 Nitrogen and ethylene are, chemically, totally dissimilar gases, but they have the same density and hence the same rate of movement. As Graham showed, it is possible to separate more or less completely a mixture of gases, if the constituents are of different densities, by taking advantage of their different rates of movement. Such an _atmolytic_ method was employed by Rayleigh and Ramsay to prove that atmospheric nitrogen contained argon.