$C12$ as a basis for a unified scale of nuclidic masses and atomic weights

There exist at present three scales of atomic masses or weights: (1) the absolute scale based on the gram, (2) that defined by taking the mass of one atom of the nuclide $O16$ equal to 16 units (the “physical scale” of “atomic masses” or “nuclidic masses”), and (3) that taking the average atomic mass of the isotope mixture of “natural” oxygen as 16 units (the “chemical scale” of “atomic weights”). Of these, only the last two are in common and extensive use. The chemical scale is indefinite to the extent of the variation in the average atomic mass of oxygen from various natural sources (some IS parts per million) resulting from variations in the relative proportions of $O16,$ $O17,$ and $O18.$