ABSTRACT
In 1914, Rutherford and Andrade determined the γ-ray spectrum of radium B and radium C by the photographic method, using the same crystal of rock-salt used by Moseley. By extrapolation from Moseley's law which had just been published, it was concluded that this natural L spectrum must be accorded to an atomic number 82. This conclusion was strengthened when Siegbahn determined directly the X-ray spectrum of ordinary lead and found values for the strong lines in close accord with those found in the γ- ray spectrum. With increase of accuracy of the measurements, it is possible to decide from a study of the excited spectra whether the number 82 or 83 is involved. To confirm the accuracy of the measurements and to substantiate that the natural L spectrum of radium B is distinct from ordinary lead bombarded by electrons, they photographed the same experimental arrangement, a slit of the same width being substituted for the radon tube.
