Results of Crystal Analysis, 69 



number of analogous dioxides which show quite a different 

 -crystal form; and in some cases the very \ same dioxide 

 occurs in several forms. Thus the titanium dioxide (Ti0 2 ) 

 is found in three modifications, viz., the two tetragonal 

 forms rutile and anastase, and the rhombic bipyramidal 

 modification brookifce. 



Generally we should expect that the determination of the 

 crystalline structure of isomeric substances would lead to 

 important results, and especially in cases where we know 

 the energy of transformation from one modification to 

 another ; for this energy should equal the difference of 

 potential energy of the system of atoms in the two modi- 

 fications ; and from this equality we may expect to be able to 

 draw valuable information with regard to the law of the 

 forces acting between the atoms, or the forces which con- 

 stitute the chemical binding. 



The present paper will deal only with the determination 

 of the structure of the zircon group ; but I hope to be able to 

 extend the investigation also to the other modifications, and 

 to treat the more general atomic problem mentioned above. 



The specimens of zircon crystals at my disposal had only 

 the faces (110) and (111) well developed. The rutile and 

 kassiterite crystals, of which we had very fine specimens, had 

 the faces (100), (110), (111), (101), but no face parallel to 

 the base (001)*. 



In cases where the crystal has no face parallel to the 

 planes from which the reflexion is to be found, we have 

 generally f been able to find an edge parallel to the plane in 

 question, and the crystal has been mounted with this edge 

 horizontal and the reflexion-plane vertical. 



In this way the reflexions from the zircon planes (100), 

 (101), (001), and the plane (001) of rutile have been deter- 

 mined f. In this case, however, we cannot claim quite 

 the same accuracy for the glancing angle and the intensity 

 as when the reflexion is found from a plane crystal surface ; 

 but still the accuracy will be sufficient for our present 

 purpose. 



§4. 



The results of observations are given in Table II. and 

 graphically represented in fig. 2. For each of the three 

 minerals the glancing angles and intensity for several orders 

 have been found for the five reflexion-planes (110), (100), 

 (111), (001), (101) t. 



* All crystals were kindly lent me "by Professor W. C. Brogger, of 

 the Mineralogical Laboratory. 



t With the exception of the (001) plane of Sn 2 4 . 



.<, 



