RECENT ADVANCES IN SCIENCE 537 



CoRLiN, A., Evidence of Moving Clusters in Kapteyn's First Drift, Astroph. 

 Journ., 23, No. 782, 1921. 



VouTE, J., First Catalogue of Radial Velocities, Naiuuvkundig Tijdschrift 

 voor Med. Indie, 80, 97, 1920. 



Shares, F. H., The Surface Brightness of the Galactic System as seen from 

 a Distant External Point and a Comparison with Spiral Nebute, Astroph. 

 Journ., 52, 162, 1920. 



Shapley, H., and Ritchie, Mary, Studies based on the Colours and Magni- 

 tudes in Stellar Clusters : xviii, The Periods and Light-curves of 26 

 Cepheid Variables in Messier 72, Astroph. Journ., 52, 232, 1920. 



PHYSICS. By L. F. Bates, B.Sc, The University, Bristol. 



Prof. W. Lawrence Bragg publishes a paper on the arrange- 

 ment of atoms in crystals in the Phil. Mag. for August 1920. He 

 shows that, if we imagine crystals to be formed by the assembling 

 of the atoms in the form of spheres of suitable diameters, each 

 sphere held in contact with its neighbours, then certain useful 

 empirical relations can be deduced. Thus, in any crystalline 

 structure a certain volume must be attributed to each atom, 

 and no two atoms can approach closer than the distance 

 representing the sum of their radii. The conception is only 

 to be considered as a " working approximation," for differences 

 of about 10 per cent, between the calculated and observed 

 distances are know^n to exist in some cases ; but it greatly 

 assists in the interpretation of the diffraction patterns obtained 

 by the passage of X-rays through the more complex crystal 

 structures. Starting with Hull's determination of the structure 

 of metallic iron, the diameter of the sphere representing the 

 iron atom is taken as 2-47 Angstrom units (2-47 A), and, from 

 his own determination of the structure of iron pyrites, Prof. 

 Bragg calculates the diameter of the sulphur sphere to be 2-05 A. 

 In this way the diameters of other atoms are calculated, and 

 a curve with " atomic diameter " as ordinates and atomic 

 number as abscissae is plotted. The periodic curve obtained 

 is similar to the Lothar Meyer curve of atomic volumes ; the 

 alkaline metals and alkaline earths possess the greatest 

 diameters, and the electronegative elements the least. It is 

 shown that the large diameters of the electropositive atoms 

 are consistent with the Lewis-Langmuir theory that the 

 electropositive atom does not share electrons with neighbouring 

 atoms, but is surrounded by a stable shell of electrons, which 

 repels the shells of neighbouring atoms, and thus keeps them 

 at a fixed distance. Similarly, the small atomic diameters 

 accredited to the electronegative elements correspond to 

 Langmuir's conception that these elements share electrons, and 

 hence their centres approach each other more closely. From 

 the apparent approach to a lower limiting value for the atomic 

 diameters of the electronegative elements in each period of his 



