I AM grateful to Prof. Schuster for the opportunity he has afforded by his letter (NATURE, March 13) for the discussion of the wide generalisations that have been made with regard to the position of the radio-elements in the periodic table, consequent on the recent experimental work of A. Fleck and of the theoretical suggestions of A. S. Russell and K. Fajans. The whole question is one in which it is important that there should not be any doubt as to the real nature of the evidence adduced. Prof. Schuster's criticism of my views on the subject could scarcely be more sympathetic or helpful, and can only result in a maturer outlook on this important question.