190 Professor W. Lawrence Bragg [May 28, 



WEEKLY EVENING MEETING, 



Friday, May 28, 1920. 



Sir James Crichtox-Browxe, J.P. M.D. LL.T). D.Sc. F.R.S., 

 Treasurer and Vice-President, in the Chair. 



W. Lawrence Bragg, M.A., Lang worthy Professor of Physics, 

 Manchester University. 



Crystal Structure. 



1. The examination of the structure of crystals by means of X-rays 

 has made it possible to discover the arrangement of the atoms in a 

 number of the simpler crystal forms. We owe to Laue the original 

 experiments, first published in June, 1912, which placed this 

 power in our hands. In seeking for some means of diffracting 

 X-rays and thus investigating their nature, he was led to use a crystal 

 as a diffraction grating for the rays, the regular arrangement of the 

 atoms in the crystal structure performing the same function as the 

 lines ruled on a grating. The success of his experiment has resulted 

 in investigations which have vastly increased our knowledge of 

 X-rays, of "crystal structure, and of the structure of the atom itself. 



The problem of crystal structure, which forms the subject of this 

 Discourse, has been attacked in various ways. In his original work 

 Laue obtained diffraction patterns by passing a fine beam of X-rays 

 through a thin plate of crystal, and allowing the diffracted beams to 

 fall on a photographic plate which recorded their geometrical distri- 

 bution. Though this clearly showed that the X-rays consisted of 

 electromagnetic waves of very short wave-length, diffracted by the 

 atoms of the crystal, the complexity of the resulting pattern on the 

 photographic plate made it difficult to draw conclusions as to the 

 arrangement of the diffracting atoms. A simpler method of attack 

 was realized in the X-ray Spectrometer devised in 1913 by Sir W. H. 

 Bragg. In the course of experiments with which the author was 

 associated, the structure of crystals such as diamond, sodium chloride, 

 zincblende, fluor spar, and the carbonates of the divalent metals were 

 fully worked out, the arrangement of the atoms being determined. 



Large crystals are necessary for these methods of analysis, and in 

 order to examine crystalline substances which could not be obtained 

 except as a mass of minute crystals, Debye employed another experi- 

 mental arrangement which he published in December, 1915. Instead 

 of using a single crystal, Debye passed the X-rays through a mass of 

 finely-powdered material, consisting of crystalline fragments oriented in 



