Evidence has been presented which indicates that the protective action of dogs' sera in irradiation of pancreatin solutions with ultraviolet light is the result of a competitive absorption (screening action). A similar effect is found in simple pancreatin solutions for which we may account (at least to a first approximation) on the basis of assumed homogeneous absorption by a strong competitor in the solution for the radiations having inactivating power. These observations are of interest in connection with the theory of what has been called negative catalysis, especially in view of the marked effects of small quantities of the protecting substances.

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