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SCIENCE PROGRESS 



by Moseley, no break is observable under this closer inspection 

 in the plotted trace which shows the connection between 

 frequency and atomic number. The curve is still quite smooth. 

 The earlier experiments showed a tendency for the ratio be- 

 tween the square root of the frequency and the atomic number 

 to increase somewhat for the higher numbers ; and the ten- 

 dency is maintained. It must be a real effect and not due to 

 some experimental error. 



It has been known for some time that an X-ray of any 

 frequency can be excited by an X-ray of a different frequency 

 only if the exciting frequency is the higher of the two. It 





/3 rt . 



o-i"oo All. 



of, 



0'6oo AU 



Fig. 3. 



The figure shows the K series for rhodium (atomic weight 102*9, *tomic number 45), palladium (io6'7 

 and 46), and silver (107 '9 and 47). As the atomic number increases the lines move up towards the higher 

 frequencies, but their relative positions and intensities do not change. 



had also been shown (W. H. Bragg, Phil. Mag., March 191 5) 

 that in a certain case at least the K series of any element could 

 only be excited by X-rays of frequency at least as great as 

 the greatest frequency of the series. For example, consider 

 the K spectra of the elements Ag, Pd, Rh, shown diagram- 

 matically in fig. 3. In the experiments referred to it was 

 found that Pd had no special absorption for any of its own 

 radiations, nor even for the a-rays of Ag. But the /3-ray of 

 silver was very highly absorbed by Pd ; and it will be ob- 

 served by reference to the figure that the wave-length of this 

 ray is just less than the least wave-length of the K series of Pd. 

 From other sources it is known that a sudden increase in 



