vroduccs in the Double Refraction ofCnjUals. 4-15 

 A, No. 2. B, No. 2. C,No.2. 

 Real vavbtion of tleviatlon -1' 47" -3' 57" -2' 52" 

 Real variation of refract-) _j_3Q,,.Q _i' li" —40" 



ing angle J r • 



Whence we obtain the following indices of refraction : 

 A No. 2. B, No. 2. C, No. 2. 



l'-534-16 1-694.21 1-68976 



At the ordinary temperature of the air they were 

 1-534-78 1-69510 1-69058 



Thus the diminutions which an increase of temperature of 

 64° has produced in the indices of refraction .n the spectra 

 polarized perpendicular to the axes of crystallization, are 



-0-00062 -0-00089 -0-00082 



The double refraction of arragonite thusappecns to decrease 

 a IMe -with the tempemture, because die retractmg power 

 fn e direction of the axis A has diminished in a smaller ratio 

 than that according to the axes B and C. In other respects 

 li'aVonl comnon^ itself quite differently from calcareous 

 smr° the axis A of arragonite obviously corresponds with 



hTaxsof crystallization"of the spar; but notwithstanding 

 th^s the refractino- power in this direction diminishes n. the 

 fo L ant o^he^o^ increases in the atter ; besides 



that in the d rection perpendicular to the axis A, f^e refract- 



n' power diminishes considerably in arragonite whilst, on the 

 contrary, it undergoes almos^no_change in Iceland spai. 



Observations on the preceding Paper. 

 The optical readers of this Journal will, we are «"re Join 

 with us in expressing our obligations to Professor Rudberg 

 for the accurate and%aluable observations contained in the 

 preceding communication, which he has been so knul as to 

 transmit to us. The subject is entirely new, and we t ust that 

 he will extend his researches to other miiiera s and also to 

 artificial salts. The influence of heat m modifying the re- 

 ft^ctive power of uncrystallized solids, such as glass, gums, 

 &c of fluids, such as water, oil, 8cc. ; ot fluids with circular 

 pokri ation, s'uch as oil ofturpentine, 8cc. ; and of nnnerrds, 

 &c. belonging to the tessular system, such as rock salt, alum, 

 &c.-mcr?t the attention of Professor I^^dberg. 



n reference to the important obse.vat.on ot M.Arago tha 

 the refractive power of water gradually increases while it 

 asses from that of its maximum density to that ol congela- 

 [ion we be.r leave to (uiote the following observations* 



"'Whenihe writer Lf this article had the pleasure of seeing 

 • Art. KxPANSioN, Kdinb. Encyclopoidia, vol. ix. p. 257- 



