EXPERIMENTS with a double mass-spectrograph have been described recently by one of us and R. Press1. These show conclusively that bombardment of a gas—contaminated surface with positive ions of one species can produce a spectrum of negative ions. In this work particular attention was directed to the process involved. We have now extended these experiments, in the first place, to identify some light ions, the identity of which was not definitely established in the earlier work. By calibrating the massspectrograph with a Kunsman alkali ion source, used in such a way that the magnet current did not have to be altered from the value used during the negative ion runs, we have identified accurately the mam negative ion peaks, with results in general accord with the early work of Woodcock2 (which we had previously overlooked), and with that of Arnot and Beckett3.