3^6 R. W. G. Wyckoff— Crystal Structures of 



The arrangement of two molecules of calcium carbon- 

 ate in the unit of structure according to (g) is shown in 

 &g. 13. 



It now remains to determine, as accurately as may be, 

 the position of the oxygen atoms, to place calcium and car- 

 bon in the correct one of their two possible positions and 

 to compare the final structure as thus obtained with the 

 results of experiment. 



The first of these objects is best attained by considering 









Fig. 14. 















/ 3 X 









+ 







/ / \\ 6 









A 















M j 



P / 







4 / \ i 









L //" 

 1 n^^ 















T \\/" 















II Vv< 















o V\ 















E M 



















7 



A 2 yW 

 / 5 J^-^ 









1.0 1 



1.1 



1 



1.2 



1.3 



1.4 



1, 



u— 



the planes having all odd indices or two even and one odd 

 indices, wherein the reflection is due solely to oxygen 

 atoms. 26 In the photographs, consideration has been 

 given not only to all such planes as appear but also to all 

 planes of this sort which could possibly give a reflection 

 in any of the photographs. Those of part B (Table 



26 It should be pointed out that if the law connecting the atomic number 

 of the reflecting atom and intensity of reflection were definitely known, and 

 if it were possible to photometer the reflections with accuracy, the position 

 of the oxygen atoms could be obtained probably with very much less work by 

 using some of the planes with one even and two odd indices. Conversely, 

 from a knowledge of the exact position of all the atoms (as determined here 

 with the aid of the planes reflecting only oxygen atoms) it should be possible, 

 as soon as an accurate photometering can be made, to get information con- 

 cerning the law of reflection itself. 



