On the Nature of the Ions in Air and in Carbon Dioxide

Mobilities of ions of air and C${\mathrm{O}}_2$ in initial and final stages.—It has previously been shown that in air the positive ions have an initial mobility equal to that of negative ions, 1.87 cm/sec./volt/cm, which decreases in about 1/50 sec. on the average to 1.36 cm/sec./volt/cm. Using improved apparatus with an air speed of 20 m/sec. and fields of over 1000 volts/cm., curves were obtained showing clearly the presence of both initial and final ions. These are the same whether $\alpha$ or $\beta$ rays are used to produce the ions. Negative ions show only one mobility. C${\mathrm{O}}_2$ ions gave curves which, within experimental error, agree with those for air, indicating one negative and two positive ions, but the time of transition to the final positive stage is somewhat shorter for C${\mathrm{O}}_2$. These results are best explained by assuming that in these cases ionization involves merely the detachment of an electron from a molecule, leaving a positive mono-molecular ion which soon attaches itself to a neutral molecule forming the final somewhat unstable bimolecular positive ion, while the electron almost immediately joins a neutral molecule forming a stable negative mono-molecular ion. Dissociation seems to play only a small part. It is concluded that single molecules singly charged, positively or negatively, all have about the same mobility which is independent of mass and molecular volume, and that a singly charged cluster of two or more molecules, not forming a single molecule, has a smaller mobility.