MICRO-ANALYSIS. 



^2>7 



respectively. In addition, in a very few cases a terminal edge 

 was observed, making an angle of 75 with the c axis. If we 

 assume that these twins are seen in the same aspect and that 

 this is the macropinacoid, i.e., that all the twinning planes and 

 the termination are brachydomes, we have an axial ratio b:c^= 

 1:0.2734, and the three kinds of twins may be explained as 

 follows : 



Calculated. 



1. Twinning plane 043, angle cAc' = 4o 14' 



2. Twinning plane 052, angle cAc' = 68 40' 



3. Twinning plane 041, angle cAc' = 84 52' 

 Termination (on), angle on A 011 = 149 24' 



Observed. 

 ,0 



40 

 69 



150' 



Fig. 33S- Digitoxin: sphero-crystals from an alcoholic solution. 



The twinning planes and the termination may, however, be 

 pyramids with the same a: b ratio. 



Optical properties : The extinction is straight. In the ma- 

 jority of cases, the length is the direction of least refractive 

 index, l)ut in others the reverse is true. This would indicate 

 that \y =c. This is confirmed by the interference figure which 



