This is a new table that brings the rare earths in from the cold. In particular, it predicts a dramatic role for thulium (a hitherto obscure element) as between silver and gold in group I. The paper thus ended with a prediction and a hypothesis to test: that there exists monovalent thulium, either Tm[+1] analogous to aurous oxide or Tm[-1] analogous to caesium auride.

E. G. Marks and J. A. Marks

A refreshing look at the Periodic Table