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52 The spontaneity and the indefinite duration of the emission by steel suggests the idea of assimilating it to the radiant properties of uranium, discovered by M. H. Becquerel, properties which the bodies since discovered by M. and Mme. Curie, viz. radium, polonium, etc., exhibit with so much intensity. Nevertheless, "N" rays are certainly spectrum radiations; they are emitted by the same sources as spectrum radiations; they are reflected and polarized, and possess well-defined wave-lengths, which I have measured. The energy which their emission represents is most likely borrowed from the potential energy corresponding to the strained state of tempered steel; this expenditure is doubtless very slight, since the effects of the "N" rays are likewise slight, which explains the apparently unlimited duration of the emission.

An iron plate, bent so as to impress on it a permanent deformation, emits "N" rays; but the emission ceases after a few minutes. A block of aluminium, fresh-hammered, behaves in an analogous manner; but the time of emission is even shorter. In these two cases the