SOME CONTEMPORARY ADVANCES IN PHVSlCS-lll 297 



frcqufiuifs of the /.-series for tliese elements, frequency being trans- 

 lated into eciuivaienl \oltage by the same relation as above. As 

 for the excitation-potentials of the heavier elements, few measure- 

 ments on potentials of the L class have been made, and very few 

 indeed upon potentials of the M class — not nearly enough for an 

 extrapolation. The deficiency is partially compensated by calcu- 

 lating the L and .Uexciiation-pntcntialsfrom the /sT, Land iUemi.ssion- 

 frequencies — an elaborate process, requiring a good deal of care in 

 measuring and properK- interpreting the various emitted rays. In 

 this manner the potentials for the group of five M levels have been 

 estimated for the various elements from the ninety-second down to 

 the fortieth, and Horton has attempted to link onto them certain 

 excitation-potentials which he and others observed when bombarding 

 elements between number 20 and number 30 with electrons (Fig. 13). 

 The curves must be supposed to bend, somewhere between the thirtieth 

 and the fortieth elements; it is in this region that the M electrons 

 pass from the status of deep-lying to the status of shallow-lying 

 electrons. The excitations and emissions involving the shallow- 

 lying electrons of the heavy atoms form a complicated system, of 

 which the study has scarcely been begun, and will certainly prove 

 perplexing. When research in this field is completed, each of the 

 excitation-potentials and each of the emission-frequencies of every 

 kind of atom will be entered upon curves, each of the curves corre- 

 sponding to a definite and definitely-pictured process of rearrange- 

 ment in the atomic electron-system, and extending over all the atoms 

 of the periodic table which can be theatres of that process. This 

 achievement may be reserved for a later generation. 



LITER.VrURE 



F. S. Brackett: Phys. Rev. 20, pp. lU-112; 1922. 



British .Association symposium "Spectra of the Lighter Elements"; Nature 112, 



pp. 217-224; 1924. 

 M. de Broglie and G. Friedel: C.R. 176, pp. 738-740; 1923. 



G. Dejardin: C. R. 176, pp. S94-897; 1923. ibid. 178, pp. 1069-1071; 1924. 

 P. D. Koote and F. L. Mohler: Origin of Spectra (Chemical Catalog Co., 1922). 

 J. Franck: ZS. f. Phys. 11, pp. 155-160; 1922. 



H. Fricke: Phys. Rev. 16, pp. 202-215; 1920. 



G. Hertz: ZS. f. Phys. 18, pp. 307-316; 1923. Xaturwiss. 11, pp. 778-779; 1923. 



J. Holtsmark: Phys. ZS. 2i. pp. 252-255; 1922. 



F. Holweck: .Annales de Phvsique, (9) 17. pp. 5-53 (1922). C. R. 173, pp. 709-712; 



1922. IC. R. 176, pp. 57(>-573; 1923 (reflection of X-rays). 

 J. J. Hopfield: Phys. Rev. 18, p. 327 (1921i. 20, pp. 573-588 (1922). 

 F. Horton, I'. .Andrewes, .A. C. Davies: Phil. Mag. 46. pp. 721-741; 1923. 

 A. L. Hughes: Phil. .Mag. 43, pp. 145-161; 1922. 

 E. H. Kurth: Phys. Rev. 18, pp. 461-476; 1921. 



