1908-9. ] ’ CRYSTALLIZATIONS OF ERYTHRITE FROM COBALT. 445 
Crystal. Forms present. 
I b, v, Ww, y 
2 b, m, w, Vv, y 
3 b, w, v, y 
4 b, m, w, v, y 
5 b, m, w, Vv, a, X, y 
6 b, w, v, a, y 
CRYSTAL HABIT. 
Only one habit was observed in these crystals. They are charac- 
terized by broad, side pinacoids <0103, a large rough negative hemi- 
orthodome <1043, a small bright, positive hemi-orthodome 1013 and 
the positive hemi-pyramids <1113. The prismatic faces are frequently 
striated. 
For comparison several crystals fram the Schneeberg region were 
examined. These crystals are characterized by the occurrence of the 
positive hemi-pyramids <1113 and the positive hemi-orthodome <1013. 
Other terminal faces were not observed. In the Schneeberg crystals, 
as in those from Cobalt, the pinacoid <o103 is the most highly devel- 
oped crystal form. 
The habit of crystal 5 is represented in the drawing on Plate 
No. I. 
The instrument with which these readings were made is a two circle 
reflecting goniometer of the Goldschmidt model. This, together with 
instruction in the calculation and gnomonic projection of the forms 
based on these measurements have been described by Goldschmidt* 
and also by Palache.t+ 
In the monoclinic system the formule for obtaining the elements 
from the readings ¢ and g on the vertical and horizontal circles are :— 
x’=sin ¢ tan 9g. y’=cos ¢ tan ¢. 
where x’ and y’ are the rectangular co-ordinates of the point. A 
*Goldschmidt :—Goniometer mit zwei Kreisen, Zeitschrift fiir Krystallographie 1893, 21, 210-232. 
Die zweikreisige Goniometer (Modell 1896) und seine Justierung Idem 1808, 20, 333-345- 
+Charles Palache :—On Crystal Measurements by means of Angular Co-ordinates and on the use of the 
Goniometer with two circles. American Journal of Science 1896 (4), 2, 298. 
