SOME early measurements of Anderson and Nedder-meyer1 show that the number of single secondary electrons ejected by cosmic ray particles from a metal plate is in reasonable agreement with that to be expected from direct elastic collisions. In a series of photographs we have observed the traversal of a 2 cm. plate of gold by about 900 particles, all of which may be assigned to the penetrating group. With this gold plate, which has a thickness of 8.5 in the units of the cascade theory, the behaviour of electrons can be immediately distinguished from that of the penetrating particles, and so the behaviour of the penetrating rays can be examined separately. On the assumption that these penetrating rays have a mass greater than that of electrons, Bhabha2 has calculated the number of ordinary secondary electrons knocked on by collision and the subsequent cascade showers that the latter produce. The particles are considered in two energy groups, above and below 3 × 109 e. volts, previous measurements of the energy spectrum for the magnetic field and counter arrangement used having shown that 44 per cent of the observed rays have an energy greater than 3 × 109 e. volts. For convenience of the comparison with theory, the secondary electrons have been classified into those with energy greater and less than the critical energy of the cascade theory, for gold, 107 e. volts.