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MS. recerved 27th June 1950A&'STRACT. The exposure was made a t the Jungfraujoch, altitude 3,457m., using Ilford type C2 nuclear research emulsions. The frequency of stars under thicknesses of carbon up to 220 gm/cm2 was measured, the experimental value of the absorption length of the radiation causing them being 16618 gm/cmz. The proportion of the stars due to fast ?.mesons has been evaluated and, on subtracting this proportion, the absorption length of the radiation responsible for the remainder is found to be 1431 10 gm/cma. The energy distribution of the stars having from 3 to 20 tracks is found to be of the form dE/EZaB, with no apparent discontinuity. The frequencies of slow n-and p-mesons at various depths is reported. The n-mesons exhibit a transition effect which is interpreted in terms of their production in stars occurring in the carbon absorber. The ratio of negative to positive n-mesons is found to be 3.1 to 1. 1. I N T R O D U C T I O NN a recent paper (George and Jason 1949 a) an account was given of experiments with Nuclear Research photographic emulsions exposed under I lead absorbers to the cosmic radiation at the Jungfraujoch at a height of 3,457 m.In the present paper we describe similar observations on emulsions exposed under carbon absorbers, also at the Jungfraujoch. For brevity we shall continue to refer to the strongly interacting component of the cosmic radiation causing the disintegration 'stars' as the ' N-radiation '.A comparison of the absorption of the N-radiation in lead and in air had led us to the conclusion that the N-radiation consisted of nucleons. From the cloud chamber observations of Blackett, W. M. Powell and Hazen, it was thought that protons formed only a small part of the N-radiation, and that most of the stars were therefore produced by cosmic-ray neutrons. A similar conclusion was also published by Bernardini, Cortini and Manfredini (1949). Perkins (1947) and Bielovitski et al. (1949), on the other hand, have suggested that the stronger absorption of the N-radiation in air than in lead might be due to the fact that the particles of the N-radiation were unstable. This would then give an absorption anomaly similar to that reported by Rossi, Hilberry and Hoag (1940) for the p-mesons.The present investigation was undertaken in order to decide between these Viewpoints by comparing the absorption of the N-radiation in air and carbon. The preliminary results, consistent with the nucleon hypothesis, were communicated to the Cosmic-Ray Conference at Bristol. A more detailed account of the observations is given here, together with a discussion of the transition effects in carbon observed for the 7~-and the p-mesons.

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