68 Dr. L. Vegard on 



For the simple cube lattice (a) : u = 1, 



For the cube-centred lattice (h) : n — 2, 



For the face-centred lattice (c) : w == 4, 



y3* 

 1 



2 V 3 * 

 1 

 V3* 



A lattice like that of diamond I ?i = 8, e = — — J is excluded 



on account of the normal distribution of intensities. 



Calling the glancing angles in the three cases 6 a , Ob, @c, 



we get 



sin 6 a = sin b sin C $ 



In Table I. are given the glancing angles for gold and lead 

 calculated for the three lattices, and also the observed values,, 

 which are in perfect agreement with the values calculated 

 on the assumption of a face-centred lattice. 



Table I. 





Calculated. 



Observed. 



9a. 



o b . 



9c. 



e v 



Gold 



11° 55' 

 9° 47' 



19° 08' 

 15° 38' 



7° 28' 

 6° 09' 



7° 26' 

 6° 09'-5 



Lead 





Thus it is proved that gold and lead crystals have the same- 

 lattice as copper and silver. 



§ ?). The Structure of the Zircon Group. 



The mineral zircon is a compound with the chemical 

 formula ZrSi0 4 . It may be considered as an addition 

 product of equivalent portions of the two dioxides (Zr0 2 , 

 Si0 2 ), or as the Zr-salt of an acid of Si corresponding to a 

 formula Zr(Si0 4 ). 



The zircon crystals belong to the tetragonal system of the- 

 bipyramidal class. Isomorphous with zircon are found a 

 number of substances, of which the following are the best 

 known : 



Rutile (Ti0 2 )„ kassiterite (Sn0 2 ) 2 , and thorite (ThSi0 4 ), 

 the latter being analogous to zircon. 



The determination of the structure of these substances will 

 be of special interest also for the reason that there are a 



