Dissociation of Iodine Vapour and its Fluorescence, 18 ( .) 



in the analysis of the more complex crystal structures, since 

 the conception of the atoms as an assemblage of spheres of 

 known diameters packed tightly together limits the number 

 of possible arrangements which have to be tried in inter- 

 preting the diffraction of X rays by the crystal. 



4. The physical significance of the relation is examined 

 with reference to Langmuir's theory of atomic structure. 

 From this point of view, it follows that two electronegative 

 atoms are situated close together in a crystalline structure 

 because they share electrons, and the spheres representing 

 them are therefore assigned small diameters. On the other 

 hand, an electropositive element does not share the electrons 

 in its outer shell with the neighbouring atoms, and is 

 therefore situated at a distance from other atoms so that it 

 appears to occupy a greater space in the structure. 



5: It is shown that the relation is less accurate when 

 applied to the crystals of metals, which, on Langmuir's theory, 

 consist of an assemblage of positive ions held together by 

 electrons which have no fixed positions in the structure. 



6. From the distance between electronegative atoms 

 holding electrons in common, an estimate is made of the 

 diameter of the outer electron shell in the inert gases. 



Manchester University, 

 April 1920. 



XIX. The Dissociation of Iodine Vapour and its Fluor- 

 escence. By St. Landau, B.Sc, Lecturer in Physics at the 

 Governmental Technical School, Warsaw, and Ed. Stenz*. 



I. The aim of this work. 



r l 1 HE researches of R. W. Wood on the fluorescence of 

 X the vapours of sodium, mercury, and iodine are 

 generally known ; he discovered the remarkable phenomenon 

 of optical resonance in these vapours. The most complicated 

 relations were found b} r Wood f in the case of iodine vapour ; 

 the number of lines in the absorption spectrum of iodine is 

 estimated by Wood to be 40,000-50,000. Different " reso- 

 nance spectra" may be obtained, when the exciting line 

 covers different absorption lines. 



We put the following question : Is the complicated vi- 

 brating system, which corresponds to these various resonance 



* Communicated by the Authors. Presented bv Prof. L. Natanson 

 to the Polish Academv of Sciences (Cracow) 18th Nov. 1919. 

 t Phil. Mng. March"l918, p. 236. 



