In order to study the energy loss of cosmic-ray electrons inlight materials where inelastic atomic collisions account for the mainpart of the loss, 10,000 cloud chamber photographs were taken in whicha thin-walled copper counter or a combination of one or two counterswith a 1/4 inch carbon plate was placed across the chamber as an absorber.The counter or counters was used at the same time to control the expansion.Only electrons of energy from 10 to 60 Mev are suitable for this purpose.These occur infrequently and only 36 particles in the case of a singlecounter absorber and 33 in the case of a counter-carbon absorber werefound suitable for accurate measurement. The observed average values ofenergy loss in Mev/cm are compared with the theoretical average value.The result shows that in the energy range considered the theoreticalformula for energy loss of electrons (no heavy tracks were included) bydirect collisions are in approximate agreement with observation. Theimportance of an experimental test of these formulae lies in the factthat the mass estimates of the mesotron so far made have been based onthe validity of these formulae.
Several heavily ionized tracks were discussed by consideringtheir ionization and range relations. One of these was found to beprobably a mesotron of negative charge. Two cases of abnormal energy losswhich can not be accounted for by ionization and radiation alone were alsodiscussed.
