20 CONTEMPORARY SCIENCE 



and Bragg, in England. The method is simply this. We 

 analyze light by a grating which consists of a series of 

 equally spaced lines on a reflecting or transmitting sur- 

 face. With such a device we can split light up into a 

 spectrum ; but we cannot do this unless the width of the 

 grating space is comparable with the wave length of the 

 light. In the case of X-rays, we had no knowledge of 

 gratings whose grating spaces were anything like as small 

 as the wave length of X-rays. In fact such gratings were 

 unknown until Laue had the bright idea of using the regu- 

 lar arrangement of the atoms in a crystal for a grating to 

 see whether that would not do the work, and it did the 

 work marvelously well. It was found that we could com- 

 pute the grating space of certain crystals from the density 

 and the atomic weight, and then from the observed spec- 

 trum find the wave length of X-rays. And now knowing 

 the wave length we can work backward and find the grat- 

 ing-space for other crystals. We are now using this 

 method for finding the positions and the arrangements of 

 the atoms in crystalline bodies. Prof. Bragg in his recent 

 book on X-rays and crystal structure has described this 

 work very beautifully. Thus a whole new field of ex- 

 perimentation has been opened up and is being pursued 

 in a great many laboratories, and with particular success 

 by A. W. Hull at the laboratory of the General Electric 

 Company. There are almost unlimited possibilities for 

 the chemist in the discovery of the exact position of the 

 atoms in any kind of crystal by this method. 



But the results of this discovery as of most of the others 

 which I have mentioned are rather insignificant when com- 

 pared with those of the ninth which I am going to men- 

 tion, namely the discovery of the relations between the 

 elements, and the extension of our knowledge of the radi- 

 ations emitted by different substances. This discovery 

 was made by a young Englishman, Moseley, only twenty- 

 six years old, who has already, unfortunately, fallen a 

 victim to this juggernaut which is at the present time 



