904 



SCIENCE 



[N. S. Vol. XLIV. No. 1148 



the much shorter ones which may be ex- 

 amined with the aid of nature's diffraction 

 gratings, crystals, which have made the 

 study of X-ray spectra possible. 



Of all the discoveries of recent years, 

 that of the wave nature of the X-rays and 

 of a practical method of examining their 

 spectra is the most remarkable and the 

 most important, for it has revealed to us 

 the most fundamental radiations of the 

 elements and has given us a glimpse into 

 the very heart of the atom. In quick suc- 

 cession Laue and his pupils demonstrated 

 the diffraction effects produced by crystals, 

 the Braggs showed how reflection might be 

 employed to isolate waves of different 

 lengths by a principle similar to that pro- 

 ducing colors of thin plates, but of far 

 greater resolving power by reason of the 

 greater number of effective reflecting sur- 

 faces, and Moseley photographed many 

 characteristic spectra by an extraordinarily 

 simple method. He found that the prin- 

 cipal lines in the spectra of a large number 

 of elements were connected by a remark- 

 ably simple relation, namely that the 

 square roots of the frequencies are pro- 

 portional to the ordinal numbers, which in- 

 crease by one in passing from one number 

 of a periodic group to the next. When 

 there are anomalies between the atomic 

 weight and the place of an element in a 

 group, this anomaly disappears when the 

 atomic number rather than the atomic 

 weight is considered. This work has been 

 extended by others, notably by Siegbahn 

 and Friman, to include nearly all the 

 known elements between sodium and 

 uranium, inclusive, with the result that all 

 the atomic numbers between hydrogen and 

 uranium are accounted for, with the ex- 

 ception of six gaps. As interpreted by 

 Bohr's theory, the ordinal number which 

 determines the frequency is the excess 

 number of positive elementary charges in 

 the nucleus, and these results are, there- 



fore, in complete harmony with the theory 

 of the nuclear atom developed by Ruther- 

 ford, van den Broek, Soddy and others. 

 The comparison of the X-ray spectrum of 

 lead obtained by Siegbahn with the gamma- 

 ray spectrum of radium B obtained by 

 Rutherford and Andrade shows the iden- 

 tity of ten of the principal lines. This 

 strikingly confirms the accepted theory of 

 isotopes, or elements of different atomic 

 weights, which are chemically and spec- 

 troscopically alike because they have the 

 same resultant nuclear charge. 



The positions of the principal lines are 

 consistent with Bohr's general formula, 

 but perhaps this relationship is purely for- 

 mal. But whether or not this theory ap- 

 plies, apparently we can not dispense with 

 the wirkungsquantum. In addition to the 

 characteristic X-radiation of an element, 

 there is a continuous spectrum, with a 

 sharply denned boundary on the side of 

 shorter wave-lengths. The investigations 

 of Duane, Hull and D. L. "Webster have 

 shown that this boundary is accurately de- 

 fined by Einstein's relation Ye = hv for 

 fields up to 110,000 volts. Such a simple 

 law does not hold for the characteristic 

 radiations; but Webster has shown that 

 they do not appear until the voltage some- 

 what exceeds that demanded by the Ein- 

 stein relation. The longest X-waves so far 

 discovered by Siegbahn are about 12 

 Angstrom units in length, so that there is 

 not a very great gap between them and the 

 shortest waves discovered by Lyman. The 

 investigation of this region is difficult, but 

 undoubtedly means will be found to attain 

 success. Much also remains to be done in 

 the study of details of X-ray spectra, which 

 contain many weak lines, and possibly 

 bands, which have not so far been carefully 

 examined. 



During the past ten years great advance 

 has been made in our knowledge of spec- 

 tral series. Rydberg, Ritz, Paschen, Fow- 



