Measurements are presented of the changes of curvature of β-ray tracks in a cloud chamber containing air at approximately N.T.P. situated in a magnetic field of 400 gauss. These are examined for evidence of processes involving the reversible production of electrons of rest mass a small multiple of the normal value. Indications from a study of earlier investigations, that such processes might exist, are not substantiated, and the results confirm and extend those of Bleuler. It is shown that this type of cloud chamber investigation is unsuitable for obtaining useful information on inelastic scattering of fast β-particles, and that multiple scattering and observational bias are adequate to explain the anomalies previously reported.