AN important result of the development of Moseley's atomic number rule has been the impetus it has given to the search for missing elements. It is true that later arrangements of the Periodic Table indicated that eka-csesium, eka- and dwi-manganese, and eka-iodine were missing, but there were no theoretical grounds for supposing that eka-neodymium might exist until Moseley's rule showed that element number 61 was still to be identified. Moseley's work was of inestimable value to one engaged in completing the list of chemical elements for several reasons-first, it gave definite information as to the existence and location of gaps in the Periodic Table; secondly, it gave a basis for the calculation, prior to its discovery, of the X-ray spectrum of an element and indicated a technique by which lines in that spectrum might be identified; and, finally, it originated a method of examination so searching that a mixture of two elements, so closely similar in chemical properties as to be almost insepar able, could be definitely analysed. Were it not for the work of Coster and Hevesy on the X-ray examination of zirconiferous minerals, the presence in them of element number 72 would probably be still unsuspected and hafnium (or celtium) would still be listed among the rare earths. Chemical tests made on zirconium ores had frequently indicated the non-homogeneity of zirconium, but they could not give the definite proof afforded by an X-ray analysis.
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Die Naturwissenschaften (1926)