SECTIONAL TRANSACTIONS. 



SECTION A.— MATHEMATICAL AND PHYSICAL 



SCIENCES. 



(For references to the publication elsewhere of communications entered in the 

 following list of Transactions, see page 444.) 



Thursday, August 5. 



1. Prof. W. L. Bragg, F.R.S. — Quantitative Methods in X-ray Crystal 



Analysis. 



In investigations of crystal structure it is assumed that waves in the X-ray region, 

 scattered by the atoms of a crystal, obey the laws of interference which have proved 

 capable of explaining diffraction in optical experiments. This assumption is justified 

 by the success with which crystal structures of a simple type have been analysed. 

 The strength or weakness of diffracted beams can be used in a very general and 

 qualitative way to fix atomic positions with considerable accuracy. 



Analysis may be made more precise by exact measurements of the ratio between 

 the energy of radiation scattered by a crystal and the energy of incident beam. 

 Evidence is accumulating that not only are the classical laws of interference obeyed, 

 but that the amount of energy scattered by the electrons is given by the well-known 

 electro-magnetic equations. This assumption has now been tested for a large number 

 of crystals. 



Crystal analysis assumes quite a new aspect when precautions are taken to obtain 

 rehabie measurements of intensity of diffraction. The distribution of scattering 

 matter (electron density) throughout the crystal, and surrounding the atoms, can be 

 deduced from the results, and structures of a complex type can be analysed. Though 

 it is possible with rough estimates of intensity to analyse crystals with five or six 

 parameters, and this has been done in several cases, such analyses are tedious and 

 somewhat uncertain. When exact measurements of intensity are available, crystals 

 with a number of parameters expressed in double figures present no great difficulty. 

 We seem to be within measurable distance of achieving the ideal position when the 

 structure of a crystalline body can be deduced as directh' from the X-ray data, as 

 can the form of a small object from its microphotograph. 



In using the classical laws to explain X-ray diffraction we are confronted again 

 by their incompatibility with the quantum laws for the interchange of energy between 

 waves and matter. This does not necessarily invalidate the conclusions as to the 

 distribution of electrons in the atoms drawn from the X-ray data, for in so many 

 other cases the classical laws give the right solution, although there is such strong 

 evidence that their fundamental interpretation requires to bo modified. 



2. Dr. J. E. Lennard- Jones. — The Forces between Ions in Crystals. 



The first step towards the correlation of the physical properties of crystals lies in 

 the determination of the forces between the constituent atoms and ions. The size and 

 structure of the crystal cell, for instance, is conditioned by the forces of attraction and 

 repulsion. In certain simple polar crystals of the rock-salt type the forces of attraction 

 may be regarded as known (being electrostatic), and so the forces of repulsion alone 

 require investigation. 



In some recent researches it has been shown that the forces between certain ions 

 in crystals can be correlated with the forces between the inert gas atoms. The latter 

 have' been found by the methods of the kinetic theory, and thus the properties of a 

 crystal have been co-ordinated with those of a gas. 



Theoretical calculations have been made of the interatomic distances of a large 

 number of crystals which are in satisfactory agreement with the observed values. The 

 1926 Z 



