ABSTRACT
Beginning in 1907, J. J. Thompson investigated these rays by the so-called “parabola method.” His results led him to suspect there were two isotopes of neon of atomic weights, 20 and 22. This was definitely confirmed in 1919 by Aston by the use of the first mass spectrograph. The importance of the mass spectrograph is strikingly shown in the case of the uranium-lead method. Prior to Nier work there was some uncertainty in all of the uranium-lead measurements because of the question of the amount of primary lead present. Geochemical separation of uranium and lead takes place so efficiently that usually the amount of ordinary lead present in the crystal of a radioactive mineral is much less than that of radiogenic origin except in the case of very young minerals.
