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THE POPULAR SCIENCE MONTHLY 



terference or diffraction effects in their passage through crystals, thus 

 proving definitely that the X rays consist of very short waves analogous 

 to those of light. W. H. Bragg and W. L. Bragg and Moseley and 

 Darwin found that the reflection of the X rays from crystals provided a 

 very simple method of measuring the wave length of the X rays when 

 the spacing of the atoms in the crystal is known. If the X rays give a 

 spectrum containing some bright lines, the wave-lengths of the latter 

 can be simply determined. The work of Barkla has shown us that an 

 X radiation, characteristic of each element, is excited under certain 

 conditions when X rays fall upon it. The penetrating power of this 

 characteristic radiation increases rapidly with the atomic weight of 

 the radiator. In heavy elements, another type of characteristic radia- 

 tion makes its appearance. These two types of characteristic radia- 

 tion have been called by Barkla the " K " and " L " radiations respec- 

 tively. These radiations can be excited either by X rays of suitable 

 penetrating power or by direct bombardment of the element by cathode 

 rays in a vacuum tube. Moseley made a systematic examination of the 

 X-ray spectra of a great majority of the elements. For this purpose, 



Fig. 14. X-ray Spectra of Successive Elements (Moseley). The additional lines 

 in spectrum of Co and Ni are due to impurity. Brass shows the combined 



spectra of copper and zinc. 



the elements examined were bombarded by cathode rays, and the spec- 

 trum of the radiation examined by reflection from a suitable crystal. 

 He found that the spectra of the " K " radiation from elements varying 

 in atomic weight from aluminium to silver were all similar in type, con- 

 sisting mainly of two strong lines. 2 An example of the spectrum ob- 

 tained for a number of successive elements is shown in Fig. 14. It is 

 seen that with increasing atomic weight, the wave-lengths of the corre- 

 sponding lines diminish, not irregularly but by definite and well marked 

 2 In later work Eawlinson and Bragg have found that each of these linea 

 is in reality a very close double. 



