VI. 



Proceedings. \_Novemher 4th, 1921. 



When X-rays fall on an atom, it is supposed that each election 

 of the atom itself becomes a source of scattered X-radiation. 

 The waves scattered by the electrons in the direction of the 

 incident light will all be in phase, and the total amplitude 

 scattered in this direction will be proportional to the number of 

 scattering electrons. If the electrons in the atom lie at distances 

 from the nucleus comparable with the wave-lengths of the 

 X-rays, the waves scattered from the different electrons in any 

 direction making an angle with that of the incident radiation 

 will not be in phase, and the amplitude of the resultant wave 

 will be correspondingly smaller. If we could measure the 

 intensit}^ of the radiation scattered in different directions by an 

 atom, we could obtain the diffraction pattern for the atom and 

 thus get an idea of the distribution of the electrons throughout 

 its volume. 



In the case of a crystal we are able to obtain some of the 

 points on the diffraction curve due to the average distribution 

 of electrons in a very large number of the atoms of which it is 

 composed, by measuring the intensity of reflection of X-ra3^s 

 from its various planes. The necessary measurements have 

 been made for the atoms of Chlorine and Sodium, using crystals 

 of rock-salt. The method of making these measurements was 

 described. 



The results indicate that in the case of the radiation employed 

 (the Ka doublet of Rhodium) we are justified in assuming that 

 each electron in the atom scatters independently. They show 

 also that the atoms of sodium and chlorine have spherical 

 symmetry, or that if the individual atoms have different pro- 

 perties in different directions, they are orientated at random in 

 the crystal, so that the " average atom " has spherical sym- 

 metiy. Distributions of electrons which will account for the 

 observed curves are discussed. It is found for both atoms, that 

 no arrangement in which the eight outer electrons all lie on a 

 spherical shell at the same distance from the nucleus will give 

 a curve of the type observed. The results suggest that on the 

 average a few electrons, say three or four, lie in the region near 

 the edge of the atom, and that the main concentration of elec- 

 trons is much closer to the nucleus. 



Joint Meeting. 



Joint Meeting of the Society, the Manchester Classical Asso- 

 ciation and the Textile Institute, November 4th, 192 1, at the 

 Textile Institute. 



