Sir David Brewster's ^o//V^ of a new Mineral. 133 



current producing these was in the same direction as in the 

 experiments just described. 



[To be continued.] 



XXI. Notice of the Optical Properties of a new Mineral sup- 

 posed to be a Variety of Cymophane. By Sir David Brew- 

 ster, K.H., F.R.S. 

 TT AVING just received from my friend Mr. Nils Norden^ 

 ^ -■■ skiold of Helsingfors, a specimen of a new mineral having 

 interesting optical properties, I hasten to communicate a brief 

 notice of these to the readers of this Journal. 



Mr. Nordenskiold received specimens of this mineral last 

 spring from His Excellency Sir L. PerofFsky of St. Peters- 

 burgh. It was found in the Emerald mines near Caterinen- 

 burg in Siberia ; and it occurs in large crystals from one to two 

 inches in diameter, which are generally composed in the same 

 manner as is shown in fig. 38, plate vii., of the second volume 

 of Mohs's Mineralogy. Mr. Hartwall is at present engaged in 

 analysing the mineral, the result of which we shall communi- 

 cate as soon as it reaches us. Mr. Nordenskiold, however, 

 has ascertained that its colouring matter depends on a small 

 admixture oi oxide of chromium. 



When this mineral is seen in daylight it is of a bright green 

 colour, whereas by candlelight its colour is a pink red. Mr. 

 Nordenskiold has likewise observed, that when a compound 

 crystal is examined with a piece of tourmaline, or in polarized 

 light, one portion of it is of an emerald green colour, while 

 another is oio. faint dirty yellow colour; and that when the cry- 

 stal is turned round 60°, the part which was yellow becomes eme- 

 rald green, and vice versa. Mr. Nordenskiold adds that the 

 mineral seems to be more transparent in candle- than in day- 

 light. 



Having repeated these experiments I have found them in 

 every respect perfectly correct ; the yellow colour, however, 

 which is described as dirty, loses this character when the spe- 

 cimen is placed in a fluid, and it then appears to be intermixed 

 with red, so as to show that if the thickness of the specimen 

 were successively increased, the colour would be redder and 

 redder, and terminate in a bright red tint. 



Although Mr. Nordenskiold has mentioned that the com- 

 pound crystals resemble the starlike compound crystals of car- 

 bonate of lead figured by Mr.Haidinger in his edition of Mohs's 

 MineralogVj yet, from the optical phaenoniena, we are disposed 

 to regard the compound as consisting of three single crystals 

 united at angles of 60°, for if the united crystals were each 

 compound, the colours would change at every 30° of revolution. 



