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ROYAL SOCIETY OF CANADA 



angle, the axial plane being perpendicular to the length of the needles, 

 and liiat an optic axis emerged nearly normal to a cleavage face. 



Curie and Flamand, from a study of the optical properties of the 

 Algerian dawsonite, came to the same conclusion as regards the sym- 

 metry. They showed further that when the needles are lying flat on 

 the microscope stage, they always have straight extinction in parallel 

 light, so that they must be elongated along the monoclinic axis of 

 symmetry; and since the optic axial plane is normal to this direction, 

 the h crystal-axis must coincide with the mean axis of elasticity, and 

 the crystals should exhibit inclined dispersion — all of which, of course, 

 follows on the assumption that the crystals are monoclinic. 



In order to learn something more definite, if possible, about the 

 symmetry and optical properties of the mineral, at the suggestion of 

 Dr. Harrington I examined the specimens he had collected, of which 

 there is a fine suite both in the University collection and also in his 

 private cabinet. After some search I succeeded in finding about a dozen 

 terminated crystals, which gave on measurement the following results : — 



Symmetry: — Orthorhombic, holosymmetric class. 



a : b : c = 0.6475 : 1 : 0.5339. 

 Forms .observed : A(IOO), B(OIO), C(OOl), M(llO), D(Oll). 



The variations in angle observed are not greater than those com- 

 monly met with in measuring a number of crystals of the same sub- 

 stance, and the mean values agree sufficiently closely with those calculated. 

 The crystals are small, the slender needles being in most cases not 

 more than 1mm. in thickness. The common habit is shown in fia;. 1. 



