242 ACCOUNT OF THE NATIVE HYDRATE OF MAGNESIA, &C. 



a result which differs only a quarter of a per cent, from that of 

 Dr Bruce of New York. 



Optical Structure. — The Native Magnesia has One axis of 

 double refraction perpendicular to the laminae, and exhibits 

 the single system of coloured rings traversed by a black cross. 

 The character of its action is Positive like that of Quartz, and 

 the tints which it polarises are different from those of New- 

 ton's scale, resembling somewhat those which surround the 

 resultant axes of Selenite *. This mineral is not phosphores- 

 cent by heat. 



Distinctive Character. — The Hydrate of Magnesia is distin- 

 guished from Talc, by its having one axis of double refraction, 

 while Talc has two axes ; — by its lower specific gravity, and 

 greater hardness ; — by its marking paper with a polished line, 

 in place of a silvery one, as already noticed ; and by its solubi- 

 lity in acids. It is distinguished from the Common Mica with 

 two axes, by the elasticity of the latter, as well as by its two 

 axes j and it is equally distinguished from the less flexible 

 Mica of Kariat in Greenland, and from other micas that have 

 only one axis, by that axis being positive in the Magnesia and 

 negative in the Mica, and also by the character of the tints 

 with which the axis is encircled. 



It is distinguished from Selenite, by its having one axis of 

 double refraction perpendicular to the laminae, whereas Sele- 

 nite has two resultant axes lying in the plane of the laminae ; 

 — by the want of regular cleavages, — and by its solubility in 

 acids. 



* See Phil. Trans. 1818, p. 243. 



XVII. 



