Measurements of the pressure variation of the natural ionization in air, nitrogen, oxygen and carbon dioxide contained in a steel sphere of one foot inside diameter, made by H. F. Fruth for pressures from 1 to 75 atmospheres, were repeated with slight modifications. Results obtained corresponded with those of Fruth with the following exceptions: (1) although the pressure rate of variation of the natural ionization at high pressures was very small (0.4 to 1.0 ion/cc sec. atm.) no abrupt change in the rate of variation and no true "saturation" value of the ionization were observed; (2) no "anomalous" behavior was noted in the case of commercial nitrogen. Measurements to detect the variation with size of chamber were carried out with spherical iron ionization chambers of 7,9, and 12 in. respective internal diameters. The rate of production of ions per cc at high pressures did not vary unidirectionally with the size of the ionization chambers, but was least in the 9 in. sphere. To study the effect of the lining the experiments were repeated with oxygen in each of the spheres lined with aluminum of $\frac{1}{16}$ in. thickness, and in the 12 in. sphere lined with copper of $\frac{1}{16}$ in. thickness. The aluminum lining increased the ionization at low pressures, and decreased its pressure rate of variation at high pressures. The copper lining reduced the absolute value of the ionization at high pressures. The effect of water vapor on the natural ionization in aged, dust-free air, or upon its variation with pressure was found to be negligible. Both the absolute and the fractional variation of the natural ionization with time were found to depend upon the pressure, the average value of the former during a 24 hr. period being about twice and that of the latter being about $\frac{1}{5}$ as great in oxygen at 55.4 atm. pressure as in air at atmospheric pressure. Very slight evidence of a singly periodic diurnal variation in air at atmospheric pressure was observed. Attempts to determine the effects of slow chemical reactions upon the natural ionization in air and oxygen at atmospheric pressure led to negative results.