Residual Ionization in a Closed Spherical Steel Vessel One Foot in Diameter, for Pressures up to 58 Atmospheres.—The present work is an extension of previous experiments in which a linear relation was found between rate of ionization and pressure up to 22 atmospheres. Essentially the same method and apparatus was used as before. Results for air from 9 to 17 days old are given in curves. From 2 to 27 atmospheres, the rate of ionization increased linearly with the pressure at the rate of about 1.25 pairs of ions per c.c. per second per atmosphere change of pressure. Between 27 and 47 atmospheres the curves differ somewhat, the slope seeming to decrease with age, but above 47 atmospheres all curves are approximately horizontal, that is, the ionization was found to be practically independent of the pressure from 47 atmospheres up to the highest pressure tried. In view of the theoretical conclusions already established by Professor Swann this result clearly proves that the ionization due directly or indirectly to the action on the air of a penetrating cosmical or other radiation is extremely small, less than 0.1 pair of ions per c.c. per second at any pressure. Of the ionization produced at atmospheric pressure, about 1.25 pairs per c.c. per second must be due to radiations, either primary or secondary, coming from the walls of the vessel and having penetrations of from 28 to 47 ft. in air at atmospheric pressure; the rest must be due chiefly to much softer radiation, also from the walls. In the previous experiments performed outdoors over water instead of in the laboratory, the change of the rate of ionization with pressure was 1.56 instead of 1.25. The difference may be due to screening action by the laboratory building.