LETTERS TO THE EDITOR. and in Paris, several of them fam l-us for their atomic- weight determinations, doubt has lingered with r-egard [The Editor does not hold himself responsible for to our results for the very much more difficult case of opinions expressed by his correspondents. Neither . In the first place, no one but myself can he undertake to return, or to correspond with has been able to obtain a suitable material by which the writers of, rejected manuscripts intended for to test the question, and I, of course, can claim no this or any other part of NATURE. No notice is previous experience of atomic-weight work. In the taken of anonymous communications.] second place, there has been an unfortunate confusion The Atomic Weight of "Thorium" Lead. between my material, Ceylon , and , IN continuation of preliminary work published by a totally distinct mixed thorium and Ceylon Mr. H. Hyman and myself (Trans. Chern. Soc., 1914, . Lastly, there has been the widespread view, due :v., 1402) I gave an account in NATURE, February 4, to Holmes and Lawson, Fajans, and others, mainly 1915, p. 615, of the preparation of 80 grams of lead derived from geological evidence, that thorium-E, the from Ceylon thorite and of the determination of its of lead resulting from the ultimate change of in comparison with that of ordinary lead, thorium, was not sufficiently stable to accumulate over which proved the thorite lead to be 0'26 per cent. geological periods of time. This confirmation from