Excerpt from Equations of Flow in a Rarefied Atmosphere: June 17, 1959 The introduction of ionization opens a veritable Pandora's box of complex phenomena. To start, consider the mean-free-path which is no longer unique and does not, by itself, give a criterion for either free flow or simple continuum flow. There is a mean free-path for neutral-neutral encounters, for encounters between charged particles, and for encounters between a neutral and a charged particle. For simplicity let us ignore inelastic effects such as ionization, recombination, charge exchange, and related surface effects. We speak of free paths rather than collision times since the former are about the same size for electrons and for ions. We also choose to ignore as minor the difference between neutral-neutral and neutral-charged cross-sections. This leaves two distinct mean-free-paths, the neutral and the Coulomb. At the temperatures existing in the atmosphere, the Coulomb cross section is much larger than the neutral cross-section. This is counterbalanced by the fact that the neutral density is usually much larger than the charged particle density. The Coulomb mean free-path is always large (at least 300 meters), but the neutral mean-free-path can range from microscopic values at low altitudes to values large compared to the Coulomb mean-free-path. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.