IN their interesting note on the discovery of illinium (NATURE, June 5, p. 792) upon which the authors, Messrs. Harris, Yntema, and Prof. Hopkins, may be congratulated, I find the statement that ``there were no theoretical grounds for supposing that eka-neodymium [sic !] might exist until Moseley's rule showed that element number 61 was still to be identified''. Having devoted almost all my scientific life-since 1877-to the theoretical and practical study of the elements of the rare earths, and especially to the question regarding their position in Mendeléeff's periodic system (the object was not very popular forty-eight years ago !), one of the results of which was the decomposition of the old didymium in 1882, I arrived at the conviction that the gap between the neodymium and samarium was abnormally large. In my paper read before the Bohemian Academy and the Russian Association of Scientists in St. Petersburg in 1902, I came to the conclusion-not reached by any chemist before-that the following seven elements, possessing now the atomic numbers 43, 61, 72, 75, 85, 87, and 89, remained to be discovered. As regards element No. 61, the difference between the atomic weights of Sm-Nd=6.1, and it is greater than that between any other two neighbouring elements. It is remarkable that it is of the same order as that between the atomic weights of Mo-Ru=5.7, between which stands ekamanganese, and of Os-W=6.9, between which stands dwimanganese, recently discovered in our laboratory by Heyrovský and Dolejšek.