Typed Letter Signed, in French, one page, 5.5 x 8.5 in., on letterhead of the Faculté des Sciences de Paris, Institut du Radium, Laboratoire Curie, 1 rue Pierre-Curie, Paris (5e), dated 14 March 1927 and addressed to Professor B. Boltwood, Sloane Laboratory, Yale University. Curie thanks Boltwood for his letter of 28 February and writes that his results on ionium and the uranium series appear to confirm “very happily” the identity of ionium—which Boltwood discovered—with thorium-230, a question long occupying radiochemists; she also expresses “the liveliest interest” in his remarks on lead proportions in ancient minerals and the determination of the age of rocks. Signed “M. Curie” in ink below the text, the letter closes with the characteristic line that her daughter joins her in sending best remembrances. In fine condition. Dated five months before Boltwood’s death on 15 August 1927, the letter documents the international exchange between Curie and the Yale radiochemist whose work on uranium–lead dating helped establish radiometric geochronology.