Page:Popular Science Monthly Volume 87.djvu/138

134 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 radiation makes its appearance. These two types of characteristic radiation have been called by Barkla the "K" and "L" radiations respectively. 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,

 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 spectrum 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, consisting mainly of two strong lines. An example of the spectrum obtained for a number of successive elements is shown in Fig. 14. It is seen that with increasing atomic weight, the wave-lengths of the corresponding lines diminish, not irregularly but by definite and well marked