360 R. W. G. Wyckoff— Crystal Structures. 



reflect X-rays in an amount roughly proportional to their 

 atomic numbers. 



2. The structure of siderite is shown to be so nearly 

 the same as that of rhodochrosite as to be indistinguish- 

 able by the means at hand. Magnesite is shown to give 

 the same sort of pattern, and hence to have the same gen- 

 eral arrangement of atoms, as the other members of the 

 group. 



3. The positions of the oxygen atoms as determined 

 by the present method and by the spectrometer results, 

 are compared. The "normal" decline of intensities is in 

 surprising agreement with the reflections. 



4. The use of gnomonic projection in studying Laue 

 photographs is mentioned and a ruler is described, the use 

 of which reduces the time and labor of making such pro- 

 jections. 



5. The effect of the voltage impressed on the X-ray 

 tube upon the character of the Laue photograph is consid- 

 ered and the best conditions for operating a tungsten tube 

 for this work are stated. 



6. A criterion is suggested for determining, in the case 

 of an hexagonal crystal, whether the fundamental unit is a 

 rhombohedron or an hexagonal prism. 



7. Evidence is obtained from these crystal structures 

 to show the existence of groups of atoms, as carbonate 

 groups, in the crystal. 



8. It is pointed out that unless every atom in the crys- 

 tal is electrostatically charged, the outside electrons of the 

 atoms making up these crystals cannot be arranged at the 

 corners of cubes. 



9. Some connections are pointed out between the de- 

 velopment of faces on calcite and its crystal structure, 

 and a means is indicated of deciding the most probable 

 indices of a plane, when they are in doubt. The bearing 

 of these structures upon the question of what constitutes 

 a series of isomorphous substances is mentioned. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 June, 1920. 



