184 Dr Brewster on the Refractive Powers, and other 



real size, AB is a cavity in quartz, which is filled with a fluid, 

 excepting the vacuity a b, which may be made to move to dif- 

 ferent parts of the cavity. The fluid does not expand per- 

 ceptibly by heat, and is in all probability water. When the 

 specimen is shaken, the fluid becomes turbid, and of a whitish 

 colour, arising from a fine white sediment, which settles in the 

 lower part of the cavity. 



In a specimen of quartz from Brazil, belonging to Mr Spa- 

 den, there is a cavity with an air-bubble, about the tenth of an 

 inch long. It is nearly one-third full of a white powder, con- 

 sisting of crystalline particles, which, upon inverting the spe- 

 cimen, flow over the surface of the air-bubble like sand in a 

 sand-glass. In the specimens of quartz already mentioned in 

 page 128, as containing cavities with pyramidal summits, 

 there is only one fluid, in which there is generally an air-bub- 

 ble. These cavities often contain opaque spherical balls about 

 the ^i T th of an inch in diameter, which are distinctly move- 

 able ; and in one cavity I have counted ten of these balls, seven 

 of which roll about the cavity when the specimen is turned 

 round.* In a second specimen, spherical balls of the same 

 kind are copiously disseminated in the quartz, and also in the 

 cavities. In a third specimen, the balls occur near the sum- 

 mits of the pyramidal cavities, some of them being within, and 

 some of them without the cavity. 



In the crystallizations of ice, several phenomena occur, 

 which are intimately connected with the preceding inquiry. 

 When water is frozen in a glass vessel, the ice is often inter- 

 sected with strata of cavities, which have the same general 

 form and aspect as those in minerals. I have sometimes ob- 

 served frozen drops of dew, containing a portion of water, 

 which remained unfrozen even at low temperatures ; and I 

 have recently had occasion to examine some crystallizations of 

 ice, which presented the same fact, under more curious cir- 

 cumstances. 



* I have since opened several of these cavities by the blow of a ham- 

 mer. In a second or two the fluid was entirely gone, without leaving a 

 trace of its existence behind. The spherical balls remained in th« cavities : 

 They were not acted upon either by the muriatic or the sulphuric acids. 



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