the Oxalate of Chromium and Potash. 437 



three degrees at the apex of the prism. The incidence of 

 the broad faces upon the adjacent faces of the prism is about 

 140°, and therefore these faces are inclined to one another at 

 an angle of 180° — 148° x 2 = 64°. The crystal is terminated 

 by four minute planes equally inclined to the broad face and 

 the axis of the prism, but two of these faces often disappear, 

 and the crystal terminates in an oblique edge in place of a 

 triangular apex. 



If we call A A' the broad faces of the crystal, m, m 1 , m, ml 

 the other four faces of the prism, and o, eF r p f pt the faces on 

 the summit, the following are the angles which they form 

 with each other. 



Incidence of A upon A in a line passing "1 "o lrt / 



through the axis of the prism J 



• A upon m, and A' upon m' 148 



m upon m 64 



A upon o, and A' upon o 112 10 



A upon p, and A' upon p' 112 10 



o upon o', and p l upon p f 50 10 



A upon A / over o, d or p, pf ... 4 36 



The crystals of oxalate of chromium and potash are, ge- 

 nerally speaking, opake; and at thicknesses not much greater 

 than the twenty-fifth of an inch they are absolutely impervious 

 to the sun's rays. In this state their colour, seen by reflected 

 light, is nearly black ; but their powder is green in daylight, 

 and of a French grey colour by candlelight. In the smaller 

 crystals, which are generally the best formed-, the colour both 

 of reflected and transmitted daylight is blue, but that of can- 

 dlelight is purple. I have not been able to find any distinct 

 traces of cleavage. 



This salt possesses a powerful double refraction, which rs 

 no doubt related to two axes. In reference to the axis of the 

 prism the double refraction is negative, like that of calcareous 

 spar. The greatest refractive index is about 1*605, and the 

 least about 1*506, reckoning from a line near the boundary 

 of the blue and green rays. 



One of the most remarkable properties of this salt is the 

 difference of colour in the two images formed by double re- 

 fraction. At a certain small thickness the least refracted 

 image is bright blue, and the most refracted image bright green, 

 in daylight, or bright pink in candlelight. The blue contains an 

 admixture of green when analysed by the prism, and the green 

 an admixture of red, the red predominating over the green 

 in candlelight. At greater thicknesses the blue becomes purer 

 and fainter, and the green passes into red ; and at a certain 



