TN two recent communications to the Paris Academy of Sciences (Comptes rendus, May 22), by M. A. Dauvillier and Prof. G. Urbain respectively, very definite conclusions have been reached as to the identity of celtium with the missing element of number 72 on the Moseley classification. This discovery is of special interest to British workers, since Moseley's last work dealt with this particular problem. Prof. Urbain adds a statement on the unpublished work of Moseley on the X-ray spectra of his preparations of the rare earths. In his paper M. Dauvillier announces the discovery of certain lines in the L X-ray spectrum of celtium which show that its atomic number is 72. An improved De Broglie photographic spectrometer was- used, and the oxides of lutecium and ytterbium in a preparation of Urbain's were attached to the anti-cathode. The tube was run at a potential of 40 k.v., and nearly complete L-spectra of lutecium and ytterbium were obtained. In addition three lines of thulium were found and two feeble lines which were identified as the a: and 32 radiations of celtium. These lines (u1= 1-5618 A; /32= 1-3194 A) fall in the correct places for the element of atomic number 72, between the corresponding lines of lutecium and ytterbium. The 3j and yt lines of celtium which might have been expected are coincident with the 3'2 and y3 lines of lutecium. Reasons are given why these lines of celtium cannot be due to any impurities, such as other rare earths.