320 THE REALITIES OF MODERN SCIENCE 



X-rays, however, sources of short wave length are 

 available, and for such radiations crystals act like 

 diffraction gratings, the interference patterns of which 

 may be made visible by photographing. 



If various crystals are thus examined under excitation 

 by a given source of X-rays, very definite indications 

 of their molecular structure are obtainable. 1 Con- 

 versely, if a given crystal is used as a grating and is 

 illuminated by X-rays from different sources, indica- 

 tions may be obtained as to the frequency of the sources. 



The principle was employed by Moseley to deter- 

 mine the characteristic radiations of various sub- 

 stances when used as the anti-cathode of an X-ray 

 tube. 2 In other words, the sources were the electronic 

 oscillators of the substance. These were excited by 

 a bombardment of electrons from the cathode of a 

 highly evacuated tube. The spectral bands obtained 

 in this way are illustrated in Fig. 44. As the substance 

 of the anti-cathode is changed from arsenic (As) to 

 selenium (Se) or from rubidium (Rb) to strontium (Sr) 

 there occurs the same shift of the characteristic spec- 

 trum shown in the figure. This indicates that the group 

 of oscillators, constituting the atom, changes, as the 

 substance is changed to that next in the series of 

 elements, by the addition of the same number of oscil- 

 lators, that is by the same charge. The number of 

 positive electrons in the nucleus of the atom of any 

 element is thus to be determined from the number in 

 that of any other element by counting the number of 



*Cf. W. H. Bragg and W. L. Bragg, " X-rays and Crystal Struc- 

 ture," G. Bell & Sons, Ltd., London, 1915. 



2 Cf. Kaye, "X-Rays." Longmans, Green and Co., 1914. 



