produced in a gas, the case when the gas is traversed by rapidly moving electrons or positively charged atoms; the photographs show that in this case the electrons liberated from the atoms for the most part start so slowly that they are unable to travel an appreciable distance from their origin. For if the electrons knocked out of the atoms by these moving particles had an appreciable fraction of the energy of the particles they would produce ions themselves, and a Wilson photograph would show branches shooting out from the stem formed by the drops due to the particle itself. Such branches are not altogether absent, but they are so sparsely scattered as to show that the great majority of the liberated electrons are not set free by direct impact between the electron and the moving particle, a view which is strongly supported by the very interesting result obtained by Lenard and Becker that the velocity with which the electron is shot out from the atom does not depend to an appreciable extend upon the speed of the particle which knocks it out. The laws of ionization by these moving particles are very different from those by Röntgen rays; it is not unlikely that the electrons ejected come from the outer layer of the atom in the first case and from an inner layer in the second.
The study of the effects of collisions of electrons or positively charged atoms with other atoms—on which Professor Townsend and his pupils have done such valuable work—raises very interesting and searching questions as to the dynamics of the collisions between these minute bodies. Indeed, as soon as we begin to study the properties of the atom questions such as these arise which go to the very root of dynamics and compel us to examine the fundamental conceptions on which that science is based. It is quite conceivable that the study