416 DR BREWSTER on the Refractive Powers and other 



lathe ; and such is the symmetry and beauty of their outline, 

 that it is not easy to conceive that they are the result of any 

 mechanical cause. One of these cavities, which is unconnected 

 with the rest, resembles a finely ornamented sceptre, as shewn 

 in Fig. 2., in which the proportion and forms of the different 

 parts are executed in the finest taste. But what is most re- 

 markable, the different parts of this figure he in different planes, 

 so that, when it is seen in a direction at right angles to that of 

 symmetry, it appears merely a number of disjointed Hues, as in 

 Fig. 10. 



The inflexion of the cavities AB into the directions bC, &c. 

 and the discharge of their fluid contents at the surface ACB, 

 could only have taken place when the whole mass ACB, though 

 crystallised, had not attained its permanent induration *. This 

 opinion derives great support from the fact, that the lines bC 

 are perpendicular to the axes of the prism, and consequently lie 

 in the planes of most eminent cleavage. The. direction, there- 

 fore, in which the fluids were discharged, was that of least resist- 

 ance, a result which might have been expected. 



In the specimen now under consideration, there is a stratum 

 of fluid cavities, composed of a great number of parallel rows of 

 cavities, and remarkable for their symmetry. One of these rows 

 is somewhat like AB, Fig. 9. If we now suppose that when this 

 specimen had not acquired its permanent state of induration, the 

 fluids in its cavities were expanded by a considerable heat, the 

 fluid in one cavity would force itself into the adjacent ones, so 

 that the row of cavities AB would form one cavity, somewhat 

 like that in Fig. 6. If the cavities lay in different planes, as 

 shewn in Fig. 10., then the expanded fluid would descend to the 



* PATRIN, if we recollect rightly, speaks of crystals of Beryl in Siberia, which 

 were so soft that they broke like a piece of apple. 



