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VI. On certain Peculiarities in the Double Refraction and Absorption of Light exhi- 

 bited in the Oxalate of Chromium and Potash. By Sir David Brewster, K.H. 

 LL.D. F.R.S, 



Received January 27, — Read February 12, 1835. 



This remarkable salt was put into my hands about the end of the year 1 832, by 

 Dr. William Gregory, of Edinburgh, to whom I have been indebted for much kind 

 assistance in carrying on my inquiries respecting the action of coloured bodies in ab- 

 sorbing definite rays of the spectrum. A very brief examination of its optical pro- 

 perties was sufficient to indicate its more obvious peculiarities, and a short notice of 

 these was published at the time. Having received, however, from Dr. Gregory a 

 very fine group of well formed crystals, and having had an opportunity in the spring 

 of 1833 of observing their action upon the spectrum, both in their solid state and in 

 the state of aqueous solution, I am now able to present to the Society a general view 

 of the results which I obtained. 



The oxalate of chromium and potash occurs in flat, irregular, six-sided prisms. The 

 two broadest faces are inclined to each other like the faces of a wedge, whose sharp 

 edge is the summit of the crystal. These faces are considerably rounded, being pa- 

 rallel near the base, and inclined to each other about 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 ciystal, m, m', m, m' the other four faces of 

 the prism, and o, o', p, p' 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 through the axis of the prism 5° 10' 



A upon m, and A' upon m' 148 



m upon m 64 



— A upon 0, and A' upon o' 112 10 



A upon 7?, and A' upon y 112 10 



upon o', and 7?' upon y 50 10 



A upon A' over o,o' or p,p' 4 36 



The crystals of oxalate of chromium and potash are, generally speaking, opake ; 

 and at thicknesses not much greater than the twenty-fifth of an inch they are abso- 



n2 



