436 Sir D. Brewster on the Optical Properties of 



for by. the difference in the extent, and other circumstances 

 which have not yet been noticed, that may have modified the 

 action of the waters. These, however, may not perhaps be 

 ascertained until a somewhat laborious survey shall have been 

 made. As I see but little prospect of this, I judge it best not 

 to delay sending you this communication, because I flatter 

 myself it may give a fresh interest to Glen Roy, and lead to 

 satisfactory explanations of many phaenomena obviously the 

 result of the action of water, but somewhat puzzling on ac- 

 count of the difficulty of finding a barrier. There are num- 

 bers of examples of terraces of gravel indicating a certain 

 operation of water. In all such cases, if the geologist will 

 look upwards, and consider the effects of hills and valleys on 

 a subsiding debacle, an explanation will readily be found. 

 I am, Gentlemen, yours, &c. 

 Coul, Nov. 9, 1835. G. S. Mackenzie. 



LIU. On certain Peculiarities in the Double Refraction and 

 Absorption of Light exhibited in the Oxalate of Chromium 

 and Potash. By Sir David Brewster, K.H. LL.D. 

 F.R.S. $c* 

 r | ''HIS remarkable salt was put into my hands about the end 

 -■• of the year 1832, 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 absorbing definite rays of the spectrum. A very 

 brief examination of its optical properties was sufficient to in- 

 dicate 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 ob- 

 serving 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, irregu- 

 lar, 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 parallel near the base, and inclined to each other about 



* From the Philosophical Transactions for 1835, Parti. This paper was 

 received by the Royal Society, January 27, and lead February 12, 1835. 

 An abstract of it was given in Lond. and Edinb. Phil. Mag., vol. vi. p. 305. 

 See also p. 134 of the same volume. 



