some Carbonates of the Calcite Group. 



34:9 



made in spectrometer measurements. This more com- 

 plicated model 28 is useful because it gives a truer idea of 

 the relations of the different atoms to one another than 

 can be gained from a single unit cell. The relation be- 

 tween the two representations can be readily noted with 

 the aid of fig. 1. 



An attempt has been made to carry through this deter- 

 mination of crystal structure with the least possible use 

 of the two empirical laws connecting intensity of reflec- 



Fig. 15. 











~"3 n 













+ 









/ 6 / 













A 







-sY / 















M / 





















P // 





















L A//, 









































T ^\ 





















U \ 







v 4 ^ 















D N 





















E 











1 ~y 















2 





^7 













1.0 



1 



l-> 



I 





> 



i 



I.3 



u 



V 















u^ 









tion with atomic number of the reflector and the spacing 

 of like planes. In order to find out whether the carbon 

 or the calcium atoms were closer to the oxygen atoms it 

 was necessary to assume that these atoms scatter 

 roughly in proportion to their weights. This is the only 

 step of the process where such use has been necessary. 

 Otherwise it has been sufficient to make use of the theory 

 of space groups and to assume that oxygen has an ap- 

 preciable scattering power. Such determinations as this, 

 where the structure can be uniquely obtained without the 

 aid of one or the other, and especially of either, of the 

 "laws" of reflection, are of interest because of the oppor- 



28 W. L. Bragg, op. cit. ; W. H. Bragg and W. L. Bragg, X-rays and 

 Crystal Structure, p. 117, 1918. 



Am. Jour. Sci.— Fourth Series, Vol. L, No. 299.— November, 1920. 

 26 



