SECTIONAL TRANSACTIONS.— A. 311 



boundary field. These calculations seem to give a satisfactory account of the observed 

 facts. The type of the formulae obtained and the comparison with observation will 

 be described in the actual paper. 



Sir Gilbert Walker, F.R.S. — Dynamics and Sport (with experiments). 



Reports of Committees. 



Thursday, July 25. 



Prof. J. T. Morrison. — The General Circulation of the Atmosphere. 



Prof. W. DE SiTTER.^ — The Rotation of the Earth. 



Prof. A. S. Eddington, F.R.S. — Matter in Inter-Stellar Space. 



A survey is given of the present position of the problem of the cosmic cloud which 

 is believed to pervade the whole galactic system. The direct observational evidence 

 of its existence is afforded by the ' stationary ' lines of calcium and rodium observed 

 in the spectra of the more distant stars. These indicate a medium not sharing the 

 individual motions of the stars. The different parts of it are nearly at rest relative 

 to one another, except that the galactic rotation (found by Oort) affects the medium 

 in the same way as the stars. Observational evidence is supplemented by a study 

 of the physics of an extremely diffuse medium under the influence of the feeble 

 radiation due to the stars. It appears that it will rise to a temperature not far short 

 of that of the hottest stars, say 15000°. In such a medium condensations will occur 

 at an average distance apart determined by the density. Taking the average distance 

 of the condensations (diffuse nebulae) to be 150 parsecs, the density is IQ-'". If 1 per 

 cent, of the cloud is assumed to be calcium, this gives just the right absorption to 

 produce the observed strength of the stationary calcium lines. Various outstanding 

 difficulties are discussed. 



Prof. A. Ogg. — The Crystalline Structure of the Alkaline Sulphates. 



Dr. A. E. H. TuTTON, F.R.S. — Significance of X-Ray Analysis of Alkali 

 Sulphates. 



The X-ray analysis of the orthorhombic normal sulphates of potassium, rubidium, 

 caesium, and ammonium is the result of a suggestion made in 1916 by the author to 

 Sir William Bragg, and was undertaken by A. Ogg and F. Lloyd Hopwood, with 

 crystals supplied by the author. It has since been ampUfied by Ogg at the University 

 of Cape' Town. Simultaneously with the latter work, W. Taylor and T. Boyer have 

 investigated the structure of caesium and ammonium sulphates, with crystals also 

 supphed by the author, and have obtained practically identical results. All agree 

 that the structure is based on the simple orthorhombic space-lattice No. 10, with 

 four molecules R.i 8O4 to the rectangular unit cell, and that the detailed structure 

 conforms to the space group V^Jj. The positions of the metallic alkaU atoms have 

 been located as well as those of the tetrahedral SO4 groups, in which the S-atom forms 

 the centre of the tetrahedron while the four 0-atoms occupy the corners. The NH4 

 group is also found to be tetrahedral, with the N-atom at the centre of the tetrahedron 

 and the four H-atoms at the corners ; moreover, the NH4 group has been shown to 

 replace the alkali metallic atom so perfectly that in the case of the middle element 

 rubidium it fits exactly into the same space. Further, the absolute lengths of the 

 edges of the rectangular cell have been obtained for each of the four salts, and thence 

 the absolute cell-volume. 



Now it is of the highest importance that these concordant results of different 

 investigators confirm perfectly the values for the relative dimensions of the unit cells, 

 the ' topic axial ratios,' which were published long ago by the author. They further 

 confirm the extraordinarily close isostructure, the equality of cell volume, of the 

 ammonium and rubidium salts, which had been pointed out by the author, and which 



