As early as 1913, James was involved in searching for element 61, the last of the rare earth elements. At least seven times, investigators believed  that they had identified element 61. Charles James and B. Smith Hopkins from the University of Illinois both sought it. In 1926, himself close to publishing on the subject, James was asked to review a paper from Hopkins for the Journal of the American Chemical Society.  James subsequently sent his own paper on element 61 to the lesser-known Proceedings of the National Academy of Sciences to avoid any possible conflict of interest. As a result it received less attention than might have been expected. It is likely that neither chemist was aware of the others work before James reviewed Hopkins paper.  James also reviewed Hopkinsʼs book Chemistry of the Rarer Elements, saying he recommended it.  
Eventually it was determined that promethium, a radioactive element, does not form stable isotopes.  As a result, it is extremely rare and unlikely to occur in nature, but can be formed in a nuclear reactor.