Reprinted in The Physical Review - the First Hundred Years, AIP Press (1995) CD-ROM.
The proton-electron plot of known atomic nuclei shows some rather marked regularities among atoms of lower atomic number. Up to O16 a simple step-wise figure appears into which the nuclear species H2, H3 and H5 could be fitted very nicely. Birge and Menzel have shown that the discrepancy between the chemical atomic weight of hydrogen and Aston's value by the mass spectrograph could be accounted for by the assumption of a hydrogen isotope of mass 2 present
to the extent of 1 part in 4500 parts of hydrogen of mass 1. (Extracted from the introductory part of the paper.).
Related references See also J. Urey, Am. Chem. Soc. 53 (1931) 2872;
R. T. Birge and D. H. Menzel, Phys. Rev. 37 (1931) 1669;
W. H. Keesom and H. van Dijk, Proc. Acad. Sci. Amsterdam 34 (1931) 52;
Connelly, Proc. Phys. Soc. 42 (1929) 28;
Finkelnburg, Z. Phys. 52 (1928) 57;
From the analysis of the atomic spectra in a discharge tube.