174 Prof. W. L. Bragg on the 



zinc and oxygen centres is 1*97 A. The oxygen atom 

 appears to occupy a smaller space than the sulphur atom, 

 and the diameter 1*30 A. must be assigned to the sphere 

 representing it. 



5. In comparing zinc oxide and zinc sulphide, the substi- 

 tution of oxygen for sulphur decreases the distance between 

 atomic centres by 0*38 A. This holds true for a number of 

 compounds, as will be seen by the following comparison. 

 It is assumed that the compounds MgO, CaO, SrO, BaO and 

 MgS, CaS, SrS, BaS, have the same structure as NaCl. 

 This is known to be the case for MgO*, and all these com- 

 pounds fall into two isomorphous series of cubic crystals with 

 cleavage parallel to the cube faces. 



The distances given in the table are those between neigh- 

 bouring atomic centres. 



MgO 



MgS 



CaO 



CaS 



SIO 



SIS 



BaO 



BaS 



ZnO 



ZnS 





I 



)ifference. 



2-11 

 2-54 



} 



0-43 a. 



2-40 



2-77 



} 



0-37 A. 



263 

 2-99 



} 



036 a. 



2-81 

 3-20 



} 



0-39 A. 



1-97 

 235 



\ 



0-38 a. 



6. Similar relationships are shown by the alkaline halidesf 

 the molecular volumes of which form a regular series. All 

 these salts crystallize in the same form as NaCl. The following 

 table gives the distances between atomic centres expressed in 

 Angstrom Units. 



NaF ... 2-39 KF 273 



Biff -41 -40 



NaCl ... 2-80 KOI ... 3'13 EbCl. 

 Diff -17 -15 



NaBr ... 297 KBr ... 3'28 EbBr. 

 Diff -26 -24 



Nal ... 323 KI ... 352 Ebl ... 3*66 Csl ... 361 



The replacement of Fluorine by Chlorine, Chlorine by 

 Bromine, and Bromine by Iodine, increases the dimensions 

 of the structure by an approximately constant amount. 



This series is peculiar in that the replacement of Rubidium 

 by Caesium appears to diminish the dimensions of the 



* Hull, Joura. Amer. Chem. Soc. August, 1919. 



f The figures from which these results have been worked out are 

 taken from G roth's Chemische Krystallographie. 



3-28 CsCl .. 



3-26 



•16 



•14 



3'44 CsBr . 



3-40 



•22 



•21 



