136 



Tables 393,394. 

 RONTCEN (X-RAYS) RAYS. 



TABLE 393. — Mean Absorption Coefficients, 



If lo be the intensity of a parallel beam of homogeneous radiation incident normally on a plate 

 of absorbing material of thickness t, then I ^ lo e—^x. gives the intensity I at the depth x. Because 

 of the greater homogeneity of the secondary X-rays they were used in the determination of the 

 following coefficients. The coefficients A. have been divided by the density d. 



Barkla, Sadla, Phil. Mag. 17, p. 739, 1909. 



TABLB 394. — X-Ray Spectra and Atomic Numbers. 



Kaye has shown that an element excited by sufficiently rapid cathode rays emits characteristic 

 Rontgen radiations. These have been analyzed and the wave-lengths obtained by Moseley (Phil. 

 Mag. 27, p. 703, 1914) using a crystal of potassium ferrocyanide as a grating. The "K" series of 

 elements shows 2 lines, a and j8, the " L'' series several. The wave-lengths of the a and $ lines of 

 each series are given in the following table. QK: = (v/f Vo)2; Ql=(v/;/6 ^'o)'^ where v is the 

 frequency of the a line and vo the fundamental Rydberg frequency. The atomic number for the 

 K series =Qk+' ! ^°^ ^^^ L series = Ql -f 7-4 approximately. vo = 3.29X lo^^ 



Moseley's summary condensed is as follows: Every element from Al to Au is characterized by 

 an integer N which determines its X-ray spectrum; N is identified with the number of positive 

 units of electricity in its atomic nucleus. The order of these atomic numbers (N) is that of the 

 atomic weights except where the latter disagrees with the order of the chemical properties. Known 

 elements correspond with all the numbers between 13 and 79 except 3. There are here 3 possible 

 elements still undiscovered. The frequency of anv line in the X-ray spectrum is approximately 

 proportional to A (N-b)2, where A and b are constants. All X-ray spectra of each series are sim- 

 ilar in structure differing only in wave-lengths. 

 Smithsonian Tables. 



