124 Dr Brewster on the Refractive Powers, and other 



Hence we have the Indices of refraction as follows : 



m = 1.620 Topaz. - 



m" = 1.2946 second fluid. 



m' = 1.1311 expansible fluid. 

 The following Table will show the relations of the indices 

 of refraction of these two new substances to those of other 

 bodies which I have found to possess a refractive power lower 

 than water : 



TABLE of Refractive Powers lower than Water. 



Water .... 1.3358 



Cyanogen rendered fluid by pressure, - 1.316 



Ice, 1-3085 



Second new Jluid in topaz, in a cavity which 

 is filled by the other new fluid, at the tem- 

 perature of 83°, - - - 1-294.6 

 New Jluid in amethyst, which fills the cavity 



at a temperature of 833° of Fahrenheit, 1.2106 



Tabasheer, whitish, from Nagpore, - 1.1825 



Tabasheer, transparent, from Nagpore, - 1.1503 

 Do. do. another specimen, - - 1.1454 



New expansible fluid in topaz, in the same ca- 

 vity as the second fluid, whose index of re- 

 fraction is given above, - - 1.1311 

 Transparent tabasheer from Vellore, of a yel- 

 lowish tint, - - - 1.1111 

 Ether expanded into nearly thrice its original 



bulk, - 1057 



Additional Observations on the New Fluids in Minerals. 



Several distinguished foreigners, and others who have taken 

 an interest in this subject, have experienced great difficulty in 

 obtaining specimens of minerals containing the fluid cavities. 

 This difficulty has no doubt arisen from their examining the 

 well crystallized specimens which are generally found in the 

 cabinets of mineralogists. If they had broken up with the 

 hammer only a few of the rounded or imperfectly crystallized 

 white topazes from Brazil or New Holland, they could scarce- 

 ly have failed to discover, with the compound microscope, in- 



