430 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. IS 



it is evident that a satisfactory decision concerning the constancy of 

 atomic dimensions can only be made after more direct information 

 is available through studies of the crystal structures of these com- 

 pounds. 



A detailed discussion of the bearing of these results upon the prob- 

 lem of the shapes and sizes of atoms will be deferred until data upon 

 other simple structures have been given. 



The method of determination. — The structures of these alkali halides 

 were obtained from a study of their powder photographs.^ A dis- 

 cussion of all of the simple ways of arranging the atoms of the com- 

 pound RX that are geometrically possible, together with the manner 

 of calculating in a qualitative fashion the nature of the diffraction 

 effects to be expected from each of these possible arrangements, has 

 been given elsewhere.^ With the following exception this same 

 method of calculation is pursued in these determinations. 



A closer accord of the "normal decline" of intensity of "reflection" 

 with the spacing of the reflecting planes as observed by spectrometer 

 measurements upon sodium chloride and other simple crystals^ is 

 obtained by assuming that the intensity is proportional to the 2.35 

 power of the spacing instead of the simple square. The calculated 

 intensities thus obtained by writing 



fid/n) = {d/ny-'' 

 give, as the following results will show, a surprisingly close qualitative 

 agreement with the intensities of the corresponding difTraction effects 

 as estimated from the photographs. 



Since a study of four or five of the most intense lines in the spectrum 

 was in all cases sufiicient to decide between the different possible struc- 

 tures, the accompanying tables will be limited to recording the data 

 from them only. In many cases, however, the number of observed 

 lines was as great as ten or fifteen. Many lines could not be observ^ed 

 in the spectra obtained with rubidium salts and from bromides be- 

 cause of the large amount of secondary radiation emitted by rubidium 

 and bromine." The dimensions of the unit cell were obtained in each 



3 A. W. Hull. Phys. Rev. (2) 10: 661. 1917; P. Debye and P. ScherrER. Phys. 

 Zeitschr. 17: 277. 1916. 



* Ralph W. G. Wyckoff and Eugen Posnjak. The crystal structures of tlie cuprous 

 halides. Journ. Amer. Chem. Soc. (in press). 



* W. H. Bragg and W. h- Bragg. X-rays and crystal structure. (London, 1918). 



* Trouble from this secondary radiation could have been eliminated for the most part 

 by the use of a second filtering screen placed next to the photographic plate. Such a film 

 was not, however, available at the time these experiments were carried out. 



