122 Dr Brewster on the Refractive Powers, and other 



Art. XXVIII. — On the Refractive Power of the Two New 

 Fluids in Minerals, with Additional Observations on the 

 Nature and Properties of these Substances.* By David 

 Brewster, LL.D. F.R.S. Lond., Sec. R. S. Edin., and 

 Corresponding Member of the Academy of Sciences of 

 Paris. 



In a former paper, on the Two New Fluids in minerals, I 

 have given the index of refraction for the most expansible of 

 the two, as it exists in the cavities of Amethyst ; but as I had 

 not then ascertained the refractive power of the second fluid, 

 and as the principal phenomena of the two fluids were ob- 

 served in topaz, it became desirable to have an approximate 

 measure of the refractive power of both of them, as they ex- 

 ist in that mineral. As the fluid in Amethyst had never been 

 examined out of the cavity, its identity with that in topaz was 

 inferred solely from the equality of their expansion by heat, 

 so that the determination of the refractive power of the latter 

 was necessary to establish either a difference between these 

 two substances, or their perfect identity. 



In the repetition of the experiments described in that 

 paper, I succeeded in finding a cavity, whose shape and si- 

 tuation in the crystal enabled me to obtain an accurate mea- 

 sure of the refractive power of the two fluids. 



This cavity consisted of a vacuity, of a large portion of the 

 highly expansible fluid, and of a considerable quantity of the 

 second fluid, which suffered almost no change by heat. The 

 situation of this cavity in the specimen is shown in Plate III. 

 Fig. 1 , where C is a section of the cavity perpendicular to its 

 length, and inclined to the parallel cleavage planes EF, GH 

 of the topaz. 



In a room where the temperature was about 60° of Fahren- 

 heit, I fixed this specimen upon a goniometer, and I mea- 

 sured the angle of incidence at the surface EF, when the 

 light of a candle RD, incident on the vacuity, began to suffer 



* This paper is an abstract of the original one, which will appear \& 

 vol. x. part 2, of the Edinburgh Transactions' 



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