70 



MARIE BENTIVOGLIO. 



Weighting these results according to the number of 

 observations made, the value of c is 0*67189 as compared 

 with Becke's ratio 0*67232. 



Combinations. — The following table shows the distribu- 

 tion of the forms in the twelve crystals examined. Where 

 triplets and fourlings occur the table shows the aggregate 

 number of forms developed on the segments twinned to the 

 normal crystal. The table also gives the total number of 

 faces present on each crystal. Crystal IX with 73 faces is 

 the fourling shown in fig. 7. Crystal IV is the untwinned 

 type crystal. It will be noticed that the most common 

 forms are a (100), h (120), m (110), and ss (231). 



Forms and Angles. — The forms present, together with 

 their measured angles and those calculated from the axial 

 ratio are tabulated below, and the degree of error also 

 shown. The average quality symbol of the signals observed 

 is given in the second column. It will be seen that no 

 results are given for c (001) due to the indefinite signals 

 obtained, a signifies an excellent, and e a very faint signal. 

 Faces of the forms e (101) and s (111) will be seen to have 

 two quality symbols, a good and a poor one. Both forms 

 tend to be well developed and give bright signals, (when 

 the readings obtained were used to determine the crystal 

 elements), but where the twinning interferes with the 

 development of these faces they are small and give poor 

 reflections — hence the two quality symbols. 



