Elastic collisions

1. In a recent paper by Bullard and Massey on the “ Elastic Scattering of Slow Electrons in Argon,” the description they give of previous work on this subject is in many respects inaccurate and misleading. Similar faults also appear in some other papers published in the ‘ Proceedings,’ where effects obtained in collisions of slow electrons with atoms or molecules of gases are described. It is therefore desirable to make clear the objections that have been raised to these papers, by considering a few cases in detail, and to refer briefly to some of the principal experiments by which the properties of slow electrons were discovered. 2. The collisions of electrons with molecules of gases in which the electrons lose only a small part of their energy, are generally known as elastic collisions. They were first observed in the experiments on the motion of electrons in air and other gases at low pressures, which were made at Oxford. The experiments were interpreted by the ordinary principles adopted in the kinetic theory of gases. The mean energy of agitation, E, of the electrons was thus obtained from the partial pressure of the electrons. The theory of the motion was fully investigated by Pidduck. He gave a method of finding the coefficient of elasticity in the collisions of electrons with molecules or atoms of a gas, and he showed that in air the collisions were almost the same as the collisions of perfectly elastic spheres.