396 



Dr. L. Veffard on 



four successive members of these substances are given in 

 Table I. 



Table I. 





v. 



X- 



*. 



w. 



Symmetry. 



NH 4 I 



57-51 

 108-70 

 162-91 

 235-95 



3-860 

 5-319 

 6-648 

 6-093 



3-860 

 5-319 

 6648 

 7-851 



3-860 

 3-842 

 3-686 

 4-933 



Cubic. 

 Tetragonal. 



N(CH a ) 4 I 



N(C 2 H g ) 4 I.... 

 N(0 3 H 7 ) 4 I 



Analysis of NH 4 L 



§ 2. The crystals which were used for the Rontgen-ray 

 analysis had the form of small cubes with faces (100), and 

 were obtained by gradual evaporation of an aqueous solution 

 of ammonium iodide. 



The reflexion from the (100) face was easily obtained,, 

 and that of the face (110) was got from an edge formed by 

 two (100) faces. To get the spectrum from (111), we had 

 to grind an artificial surface as nearly as possible parallel to 

 the (111) face. 



The glancing angles and the relative intensities of spectra 

 of different orders are given in Table II., and graphically in 



fig.i. 



Table II. 



Face. 



Glancing angle. 

 Order ... 



Intensities. 



1. 



100 

 100 

 100 



2. 



3. 



4. 



(100) 

 (110) 



(111) 



4° 50' 

 6 51 

 4 08 



33 

 31 



90 



11 

 11 



20 



20 



The spacing d m = 3*60 x 10~ s cm. In the elementary 

 cell there is 1/2 molecule. 



We are then naturally led to assume that the atoms are 

 arranged in face-centred cubic lattices. The spacings of 

 the other two faces are also in agreement with this assump- 

 tion, for we find from the observed glancing angle : 



sin # ]00 :sin # 110 :sin ni — 1: \/2 : 1/2 v/ 3. 



(i) 



