REPORT ON PHYSICAL OPTICS. 413 



it. Sir David Brewster has observed an analogous, and even yet 

 more remarkable property, in glauberite. At the freezing tem- 

 perature this crystal has two axes for all the rays of the spec- 

 trum, the inclination of the axes being greatest in red light and 

 least in violet. As the temperature rises the two axes approach, 

 and those of different colours unite in succession ; and at the 

 ordinary temperature of the atmosphere, the crystal possesses 

 the singular property of being uniaxal for violet light and 

 biaxal for red. When the heat is further increased, the axes 

 which have united open out in order, and in a plane at right 

 angles to that in which they formerly lay ; and at a tempera- 

 ture much below that of boiling water, the planes of the axes 

 for all colours are perpendicular to their first position*. The 

 inclination of the optic axes in topaz, on the other hand, aug- 

 ments with the increase of temperature; and the variation, 

 M. Marx has observed, is much greater in the coloured than in 

 the colourless varieties of this mineral f. 



• Edin. Trans., vol. xi. ; and Phil. Mag., Third Series, vol. i. 417. 

 t Jahrb. der Chemie, vol. ix. 



