The precision measurements of the masses of the three substandards, especially that of ${\mathrm{C}}^{12}$ by different observers show a slight discrepancy well outside the probable error. The masses of all isotopes determined by the doublet method depend more or less on the masses of the substandards. Therefore Dr. Aston has expressed anxiety that some agreement should be reached on the isotopic weight of ${\mathrm{C}}^{12}$ to enable a satisfactory table to be adopted. In the present paper some of Aston's objections and criticisms are discussed in some detail as well as the checks which each author has to offer for his measurement of the principal doublet in question. At the end of the paper are given the masses of the three substandards ${\mathrm{H}}^1$, ${\mathrm{D}}^2$, ${\mathrm{C}}^{12}$ as well as the mass of ${\mathrm{N}}^{14}$ which are thought to be the most reliable at present.

• Received 28 July 1939

DOI:https://doi.org/10.1103/PhysRev.57.1155