144 C. W. Cook — Crystal Form of Alhdt Halides. 



locate it definitely. The pinacoid <?j001{ is always the pre- 

 dominating face, -is well developed, and gives good reflections. 

 The orthopinacoid a j 100 J appears as a rather narrow face 

 elongated parallel to the 6-axis. Of the prisms, 6>jl20j was 

 always found to be present and to be better developed than the 

 unit prism, although both prisms were generally present. 



Fig. 1. 



Fig. 2. 



The clinodome^jOllf is not always present, but when it does 

 occur it possesses small triangular faces giving excellent reflec- 

 tions. The hemi-orthodome a? {102 \ is always present and gives 

 good reflections. 



Type two shows a tabular development parallel to the ortho- 

 pinacoid (fig. 2). This type of development was observed 

 but once and exhibited the following forms : 



aJlOOj, cJOOlj, a:{102} 5 m{110}, o{120j. 



The close agreement between the observed and calculated 

 angles may be seen from the following table : 



g = (100) : (001) = 

 m = (100) : (110) = 

 x = (001) : (102) = 

 o = (100) : (120) = 

 p = (001) : (011) = 

 o = (001) : (120) = 



Observed 

 72° 41' 30"* 

 38° 37'* 

 38° 33'* 

 58° 4' 

 46° 52' 

 80° 45' 



Calculated 



46° 53' 

 80° 55' 



The quantity of material available was not sufficient to per- 

 mit of the determination of the exact amount of water of 

 crystallization, so that the formula which is given is based upon 

 the formula that Mitscherlich established for the sodium 

 bromide. 



In conclusion the author wishes to express his thanks to Dr. 

 E. H. Kraus for valuable suggestions and criticisms. 



University of Michigan, 



Ann Arbor, Mich., April, 1914. 



