The primary object of this investigation has been to try to discover the mechanism of the action by which the positive ions set free by hot bodies originate. It deals principally with measurements of the steady positive ionisation produced by hot platinum in various gases under different conditions. In 1901 the writer showed that the negative ionisation from hot metals could be satisfactorily explained by supposing that it was caused by the freely moving corpuscles inside the metal escaping from the surface when their kinetic energy exceeded a certain value. In the present paper reasons are assigned for believing that a view of this kind will not account for the origin of the positive ions, which seem to be liberated by an action in which the atoms of the gas play a direct part. In the cases of steady ionisation here investigated it seems probable that it is not so much the external gas as that adsorbed by the metal which is effective.