The residual ionization in aged, dry, and dust free air, ${\mathrm{H}}_2$, He, A, ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, C${\mathrm{O}}_2$, C${\mathrm{H}}_4$ and ${\mathrm{C}}_2$${\mathrm{H}}_4$ has been measured at pressures from 0 to 180 cm Hg. In order to reduce radio active contamination, a glass globe with a sputtered silver coating was used as the ionization chamber. An electrometer and compensation devices eliminated the effect of fluctuations in the battery and provided a null method that kept the guard ring at the same potential as the collecting system, thus eliminating leakage. The ionization curves show a more rapid rise in the lower pressures which ceases at from 15 to 65 cm Hg, when the alpha-rays causing it are absorbed. Above this the curves are straight lines. From these portions the increase in ions formed per cc per sec. per atm. has been determined. It is least in the case of hydrogen (2.033), and largest in the case of argon (18.59). This rate divided by the number of electrons in the molecule is approximately constant (0.855). The rather good agreement with Kleeman's beta-ray results might be regarded as an indication that the principal agency operative was of a beta-ray character.

• Received 30 December 1925

DOI:https://doi.org/10.1103/PhysRev.27.659