PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



265 



these planes and the angles at which the first order spectra occur. The heights of the 

 vertical lines represent intensities, and their magnitudes are also indicated in figures 

 underneath, in arbitrary units. Their abscissae represent the sines of the angles of 

 reflection. The numerals placed above the lines on the right of the diagram show 

 the percentages of the molecular weight lying in the various planes. 



The structure of calcite has already been investigated by W. L. BRAGG,* and fig.. 15, 

 is reproduced from his paper. In its determination use was made, as in other cases, 

 of a knowledge of the crystal symmetries. The fact that the (ill) planes contain 

 alternately calcium atoms only, and groups of CO 3 only was inferred from a 

 comparison of the (ill) spectra of the calcite series of crystals. In all cases there is 



Fig. 15. 



clearly an alternation of composition because, as in NaCl, the odd order spectra are 

 abnormally small. In chalybite FeCO 3 the odd order spectra disappear altogether, 

 implying that the alternate planes are now equally weighted. If one plane contains 

 Fe only, and the other C0 3 only, this equality does actually occur. The series being 

 isomorphous we must suppose this structure to be common to them all. 



Considerations of symmetry show that the arrangement of the carbon and oxygen 

 atoms must be as shown in the lower line of drawings in the figure. 



But there is one variable which symmetry cannot determine, viz., the distance 

 between the oxygen and the carbon atoms. This can only be found by interpreting 

 the intensity measurements. In the original paper the best measurements available 



* ' Roy. Soc. Proc.,' A, vol. 89, p. 486. 



