  The theory attributed the behaviour of gases to the motions and elastic collisions of a large number of molecules. The motions were considered to be randomly distributed in the gas, while the motion of each molecule was governed by the laws of mechanics both during and in between collisions. It was necessary to assume that molecules acted on each other only during collision, that their volume was small compared with the total volume of the gas and that the time spent in collision is small compared to the time that elapses between collisions. While the molecules needed to be assumed to be small, they needed to be sufficiently large that they could not move uninterrupted through the gas. The irregular path of a molecule through the body of a gas from collision to collision was necessary to explain rates of diffusion.