Structure of the Silver Halides. 263 



Silver chloride and silver bromide each gave the pattern 

 of a simple cube of sides 2*78 A and 2*89 A respectively. 

 The density of the elementary cube shows that one atom is 

 associated with each point of the cubic lattice. Silver iodide 

 gave the diffraction pattern of the diamond lattice, in which 

 the side of the elementary cube was 653 A; one atom is to 

 be ascribed to each point in the structure. Each iodine atom 

 is at the centre of a tetrahedron whose corners are occupied 

 by four silver atoms, and each silver atom is surrounded by 

 four iodine atoms in the same manner. 



In the cases of silver chloride and silver bromide, the 

 minimal distance between atomic centres is given approxi- 

 mately by Bragg's atomic diameters*; however, in silver 

 iodide, the distance between the centres of the nearest atoms 

 is somewhat less than the value computed from Bragg's 

 data. The comparisons are given in the following table : — 



Side of Distance apart 



„ , , Arrangement Elementary of nearest Computed from 



of Atoms. Cube in atomic centres. Bra^g's Atomic 



Angstroms. Observed. Diameters. 



Silver chloride. Simple cubic. 2 78 2-78 2"82 



Silver bromide. Simple cubic. 289 2'S9 296 



Silver iodide. Tetrahedral. 6'53 2-83 3-17 



The silver halides were precipitated from silver nitrate by 

 the corresponding potassium halides ; this gives a flocculent 

 precipitate in which the crystal particles are too small to be 

 recognized as crystals under the microscope. The X-ray 

 analysis, therefore, has given the first proof that these pre- 

 cipitates are crystalline. Silver bromide and silver chloride 

 as precipitated in photographic emulsions form crystals w^hich 

 can be recognized as belonging to the cubic system ; silver 

 iodide gives evidence of forming hexagonal crystals : these 

 forms have not yet been studied by the X-ray method. 



The powder photograph of fused silver bromide showed the 

 same diffraction pattern as the precipitated salt. 



These determinations were made for Mr. A. P. H. Trivelli, 

 who prepared the samples. The X-ray investigation of crystals 

 occurring in photographic emulsions is being continued. 



Research Laboratory, 



Eastman Kodak Company, 

 Rochester, N.Y. 



April! 1,1921. 



* W. L. Bragg, Phil. Mag. xl. pp. 108-189, August 1920, 



