296 



NITRATE OF POTASH. 



tions of its faces were parallel to the plane of reflection, a 

 bright image of the candle was seen in the middle of a mass 

 of nebulous light, exactly similar to what happens in the agate 

 when its veins are parallel to the plane of reflection. But 

 upon turning round the crystal of nitre, the bright image gra- 

 dually vanished, while the nebulous light increased ; and when 

 the edges of the. crystal were perpendicular to the plane of re- 

 flection, the bright image was extinguished, and the nebulous 

 light a maximum. When the reflected image of the candle is 

 viewed through two inclined faces of the nitre, the two images 

 vanish alternately, like those formed by all doubly refracting 

 crystals. 



A prism of nitrate of potash, having its refracting surfaces 

 equally inclined to the axis of the hexaedral crystal, possesses 

 the faculty of depolarising light ; and hence it follows, from the 

 theory of depolarisation, that the prism must, in this case, form 

 two distinct images.. 



The two neutral axes of this salt, are parallel and perpendi- 

 cular to the sides of the hexaedral prism ; and the depolarising 

 axes are parallel to the diagonals of the square base common 

 to the two pyramids which compose its primitive rectangular 

 octaedron. The least refracted image is that which is produ- 

 ced by the extraordinary law of refraction. 



The beautiful coloured rings which I exhibited to the Socie- 

 ty, as produced by the action, of topaz upon polarised light, and 

 which I have- also discovered in the agate, and in a great varie- 

 ty of other bodies *, exist also, but in a very singular manner, 

 in the nitrate of potash. 



By. 



* See Phil Trans. Lond. 1814. Part i. p. 2r8. 



