312 SECTIONAL TRANSACTIONS.— A. 



had formed his reason for non-acceptance of the interesting valency volume theory 

 of Pope and Barlow, according to which the cell- volume of ammonium sulphate should 

 be double that of rubidium sulphate. Again, these X-ray results agree absolutely 

 with the law of progression of the crystal angles, constants, and properties, with the 

 atomic weight or atomic number of the interchangeable elements, which has been 

 found universally to apply in the cases of eutropically (interchanges only within the 

 same family group) isomorphous substances of strictly analogous structure-type. 

 This law has been perhaps the main results of the author's forty years of research, 

 during which not only these simple sulphates of the alkaUes, but also the corresponding 

 selenates, which afforded precisely analogous results, and seventy-four double sulphates 

 and selenates of the monoclinic hexahj'drated series, have been investigated. In 

 these latter more complicated substances the alkali sulphate or selenate is combined 

 with the sulphate or selenate of Mg, Zn, Fe, Mn, Ni, Co, Cu, or Cd, and the crystals 

 thus afforded sixteen times over independent results for the interchange of the alkali 

 metals. Moreover, the author has likewise completed a similar investigation of the 

 perchlorates of the alkali metals K, Rb, and Cs, and of ammonium. In all these cases 

 the results were perfectly similar. 



Hence, a general law has been shown to hold, that in any unbroken strictly 

 structurally comparable series of eutropically isomorphous compounds, the whole of 

 the crystallographic morphological and physical properties are functions of the atomic 

 numbers or weights of the interchangeable family-group elements which give rise to 

 the series. All apparent exceptions, such as the alkali haUdes, have been proved to 

 be cases of broken and non-comparable series. 



Finally, this law of progression is shown to be due to the progressive change in the 

 structure of the atoms themselves, eighteen electrons, in one or two complete shells, 

 being added when potassium is replaced by rubidium, and again when the latter is 

 replaced by caesium. 



Department of Cosmical Physics. 



Mr. E. N. Grindley. — Recent Results of the Magnetic Survey of South 

 Africa. 



Sir Frank Dyson, F.R.S. — Proper Motions in the Greenwich Astrographic 



Zone. 



By comparison of recent photographs with those taken about 28 years ago proper 

 motions have been determined of a very large number of stars. Analysis of the results 

 shows the streaming in two directions of these stars, and the numbers in the two 

 directions are equal. The mean parallaxes of these stars in different galactic latitudes 

 are given and their mean peculiar velocities. 



Mr. H. C. Mason. — Lmmr Craters and the Volcanic Theory. 



Friday, July 26. 



Prof. W. H. EccLEs, F.R.S.— TAe New Acoustics. 



Prof. A. M. Tyndall. — Some Problems relating to the Mobility of Gaseous 

 Ions. 



The paper deals with the following topics : (1) Established results. (2) The 

 difference between positive and negative mobilities. (3) The effect of vapours. 

 (4) Mobility in pure gases. (5) Positive ions of short age. (6) Suggestions as to 

 future progress. 



Sir T. MuTR, F.R.S. — The Literature of Cayleyan Matrices. 



Mr. H. HoRROCKS. — The Recent Spectrum of Nova Pictoris. 



Dr. H. Spencer Jones. — Elements of the Moon's Orbit and the Solar 

 Parallax from Occultations 1753-1908. 



( 



