A study has been made of scattering of 40 protons and 160 mesons in photographic emulsions in order to determine the mass of the individual particles. The spread in the values so obtained is large, but the evidence suggests that the majority of mesons recorded by the emulsion can be identified both with the mesons, of mass ≃ 200 m_e, commonly observed in cloud-chamber experiments and with counters, and with the μ-mesons observed in the photographic plates. The slow particles producing nuclear disintegrations, σ-mesons, appear to contain a large proportion of particles with a mass equal to that of the π-mesons.