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XXIX. On the Refractive Power of the Two New Fluids in Mi- 

 nerals, with Additional Observations on the Nature and Pro- 

 perties of these Substances. By DAVID BREWSTER, LL.D. icdLJ 

 F. R. S. Lond., Sec. R. S. Edin., and Corresponding Mem- 

 ter of the Academy of Sciences of Paris. 



(Read March 6. 1826.J 



IN the Paper which I had the honour of submitting to the So- 

 ciety *, on the Two New Fluids in mineral bodies, 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, especially those which related to 

 their properties when taken out of the cavities, were observed in 

 specimens of Topaz, it became desirable to have an approximate 

 measure of the refractive power of both of them, as they exist in 

 that mineral. As the fluid in Amethyst had never been exa- 

 mined in the open air, its identity with that in Topaz was in- 

 ferred solely from the equality of their expansion by heat, so that 

 the determination of the refractive power of the latter was ne- 

 cessary to establish either a difference between these two sub- 

 stances, or their perfect identity. 



In the repetition of the experiments described in that paper, 

 and in extending my inquiries to different specimens of Topaz, I 

 sought diligently for a cavity whose shape and situation in the 

 crystal would enable me to obtain an accurate measure of the re- 

 fractive power of the two fluids. Such a specimen I have had 

 



* See Page 1. of this Volume. 



VOL. x. P. ii. 3 r 



