Cosmic rays — What physicists have learned about them

“Cosmic rays” is the name applied to the ultimate cause of that part of the ionization of the air that cannot be ascribed to any known agencies. In spite of this prosaic definition, however, seldom if ever has there been a subject of research in physics in which intrinsic importance and romantic associations so happily are combined. In view of the fundamental importance of cosmic ray research, in this article is given a review of studies made in this engrossing field from the time of early experiments with the gold leaf electroscope down to the present day of many and conflicting theories and speculations.