T he phenomena connected with the penetration of high spee d particles through matter have been a most important sourc e of information about atomic processes. The discovery of nuclear fission, which made it possible to study the behaviour of swift ions with great masses and charges, has in this respect reveale d many new interesting features, especially as regards the captur e and loss of electrons by such ions. Capture and loss phenomen a were, as is well known, first observed for a-rays, and have recentl y received renewed attention through the study of the tracks in photographic emulsions of highly charged ions of cosmic origin, penetrating into the upper regions of the atmosphere. Still, experiments on the stopping and ionizing effects of fission ions, and especially direct measurements of the charge of the ions during their passage through gases and solids, offer so far the most detailed and varied evidence as regards electron loss and captur e by heavy ions.

In a previous treatise', a general survey of the theoretical interpretation of the effects accompanying the penetration of atomic particles through matter has been given. In particular, it was attempted to account for the peculiar law which governs the energy loss of fission ions along their path by estimating th e ion charge which, on account of the displacement of the balanc e between electron capture and loss, diminishes gradually with decreasing velocity. While the stopping and ionization effects in the beginning of the path depend primarily on collisions with the electrons in the atoms of the medium penetrated, nuclea r collisions become of decisive importance at the end of the path. On the assumption that, irrespective of the substance throug h