Abstract
ACCORDING to the kinetic theory a gas consists of molecules, which may be considered as little spheres, or, more generally, as little bodies of a more or less marked degree of symmetry, about a hundred-millionth of an inch long, rushing about in all directions and frequently colliding with one another and with the walls of the vessel. The collisions with the walls produce the pressure. The molecules move with a very high velocity, some hundreds of yards per second for gases at ordinary temperature, and this high velocity deduced from elementary considerations, was a point of difficulty in the early days of the theory, critics objecting that such speeds would imply very rapid diffusion, so that, for example, the vapour of any odorous liquid should be detected by its smell at the furthest parts of a room as soon as the bottle is opened. Such criticism leaves out of account the frequent collisions, which make the path of an individual molecule a zigzag with frequent turns back on itself. The average distance between collisions is called the mean free path, and is an essential factor in all questions of diffusion and of viscous fOrces. It is about a hundred-thousandth of a centimetre for air at ordinary pressure, and is, to a first approximation, independent of the temperature. It varies, with a given gas, inversely as the pressure, so that at low pressures it becomes quite large: in air at a pressure of 1 microbar it is 10 cm., and at a pressure of 0.01 microbar, easily attained with modern technique, it is 10 metres.
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ANDRADE, E. Molecular Air-Pumps1. Nature 124, 657–659 (1929). https://doi.org/10.1038/124657a0
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DOI: https://doi.org/10.1038/124657a0