Cloud-Chamber Photographs at 4310 Meters Altitude

Cloud-chamber photographs taken at 4310 meters altitude give evidence for the existence of penetrating particles in air showers. Out of 2983 successful expansions 5.8 percent showed particles classified either as slow mesotrons or slow protons. 1.3 percent of the expansions show particles whose specific ionization increases markedly on passing through a 1.3-cm lead plate in the chamber. Twenty-seven photographs show penetrating pairs and thus indicate the production of penetrating secondaries at this altitude. Some photographs give evidence for nuclear processes associated with showers. Five photographs show electrons which may have come from the disintegration of mesotrons within the chamber.