CRYSTALS CONTAINING FLUID. 187 



and the crystal being immersed in hot water at the time, 

 it was not possible to collect the fluid for examination. 

 Since this paper was written. Sir David Brewster has in- 

 formed me that he has experimented with the flnid-cavities 

 in fluor-spar, and found they burst at a temperature of 

 150°, and that the fluid is water. 



When a precious stone contains many of these cavities, 

 which to unassisted vision look like flaws and cracks, it 

 is laid aside as imperfect and unfit for ornamental pur- 

 poses. On close examination under the microscope, many 

 of those supposed to be perfect exhibit cavities. Some 

 specimens of ruby I have examined show entangled crystals 

 and fluid-cavities, which are beautifully seen by polarized 

 light. 



For the information of those Members who may not 

 have seen specimens of quartz and topaz containing fluid, 

 I will describe their usual appearance. The cavities most 

 frequently are of an irregular figure, more or less angular, 

 the fluid generally nearly filling the cavity, a little bubble 

 being alone visible, and easily moved about as the crystal 

 is inclined in dififerent directions. If, during the examina- 

 tion under the microscope, a small degree of heat is applied 

 to the crystal, the fluid will expand and fill the cavity, 

 causing the bubble to disappear entirely ; and on cooling, 

 a number of small bubbles make their appearance, which, 

 after a time, unite. 



After the Meeting, we can try this experiment. 



How Nature operates in forming these crystals is a 

 matter of mere conjecture : time is doubtless a very im- 

 portant element ; our experiments are so limited in this 

 respect, that we cannot hope to imitate her successfully. 



Many attempts have been made to produce diamonds 

 and other precious stones by the prolonged action of in- 

 tense heat. 



When a boy, I recollect my father experimenting for a 



