128 Dr Brewster on the Refractive Powers, and other 



discharged, was that of least resistance, — a result which might 

 have been expected. 



In the specimen now under consideration, there is a stra- 

 tum 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. 7. 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. 5. If the cavi- 

 ties lay in different planes, as shown in Fig. 6, then the ex- 

 panded fluid would descend to the one immediately below it, 

 and connect the whole together as in Fig. 3. We do not 

 mean to say, that the cavities bC in Fig. 7, were actually 

 formed in this manner, because this is rendered improbable 

 by their connection with the rectilineal ones AB, but merely 

 to explain how cavities having the forms shown in Figs. 3, 4, 

 5, may have their origin from the union of a great number of 

 cavities arranged as in Fig. 7- 



When the cavities are regularly crystallized, the homolo- 

 gous sides of the hollow crystals are parallel to one another, 

 and also to those of the primitive or secondary form which 

 they resemble. In some very curious but amorphous speci- 

 mens of quartz from Brazil, belonging to Mr Spaden, the 

 hollow crystals terminate in six-sided pyramids, with flat sum- 

 mits, and the axes of these pyramids is parallel to the axis of 

 the system of polarised rings, and consequently to the axis of 

 the crystal. 



3. On tlie Condition of the Fluids within the Cavities. 



The phenomena of the expansible fluid have been so ful- 

 ly described in my former paper, that I have only a few ob- 

 servations to add upon this part of the subject. In some spe- 

 cimens of quartz, the expansible fluid seems to exert a very 

 considerable elastic force, even at the ordinary temperature of 

 the atmosphere, and when a slight degree of heat is applied, it 

 sometimes has sufficient force to burst the specimen. A very 



