DR. D. F. MARTYN, Messrs. G. H. Munro and A. J. Higgs, and Dr. S. E. Williams, in a communication which appears on page 603, show evidence that a type of ionospheric disturbance accompanies every bright hydrogen solar eruption. The main features of the disturbance are an increase of ionization in the D region and a heating effect in and below the F2 region. When the disturbances are large they cause 'fade-outs' in short-wave communication. It is concluded that these effects are due to a greatly increased emission of the hydrogen resonance line La from the eruptive area. This causes ionization of atomic oxygen in the D region, and dissociates the water vapour in the F2 region, thus raising the equilibrium temperature. Further evidence of a connexion between solar activity and short-wave radio 'fade-outs' is given in the note entitled "An Active Sun-spot" on page 616 of this issue.