That a gas may be looked upon as an association of particles—hard elastic spheres—moving backwards and forwards in right lines with great velocity, and possessing in the aggregate a very small proportion of the space through which they travel, was first conceived by Daniel Bernoulli in 1738. By means of this hypothesis he explained the direct proportionality between the density and pressure of a gas. If the gas consists of moving particles, and the pressure which it exerts on the sides of the containing vessel is due to the impacts of these particles, it is obvious that by halving the original volume of the containing space we halve the space through which the particles travel, and therefore double the number of their impacts in a given time; in other words, by compressing the gas to half its initial volume we double the pressure it exerts, which is nothing else than the law of Boyle. This conception of the nature of a gas is known as the _kinetic theory of gases_; it was further developed by Waterston in 1845, still more fully by Clausius in 1857, and was subsequently placed in its present position by Maxwell and Boltzmann.
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):
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