414 DR BREWSTER on the Refractive Powers, and other 



communicate with a single cavity in the nearest rectilineal row 

 of the radiations between which the long cavities are placed. 



They have a resemblance to lakes or rivers, whose branches 

 have been supplied from these rows of cavities ; though it is 

 more likely that the expansion of the fluid within the long cavi- 

 ties, and when the substance of the topaz was soft, forced out a 

 great number of globules, some of which continued to adhere to 

 the slender filamentous cavities from which they were discharged. 



In all the cavities of this remarkable specimen capable of be- 

 ing examined, there are found both the new fluids, with the ex- 

 ception of the long branching cavity AB, from which they had 

 escaped, in consequence of the end A being cut by the lapidary. 

 The dense fluid always occupies the filamentous branches. 



In some cases there is a breach of continuity in the branches, 

 a small part of the cavity being as it were filled up with solid 

 topaz. This fact favours very much the supposition that all the 

 rows of minute cavities had been thrown off from the great ones ; 

 though the rows of cavities on the left and lower side of the spe- 

 cimen are hostile to it. 



The plane in which these cavities lie is perfectly flat, and is 

 nearly perpendicular to the axis of the prism, the line joining 

 the two resultant axes of double refraction being parallel to 

 MN. 



2. On the Form of the Cavities containing the New Fluids. 



In a former paper I have given drawings and descriptions of 

 some of the most remarkable shapes which these cavities assume ; 

 but in the prosecution of the subject, I have met with a variety 

 of new and remarkable forms. In a specimen belonging to Mr 

 SANDERSON, and which is one of the most valuable that I have 

 seen, each cavity (see Plate XIX. Fig. 6, 7, and 8), consists of 



