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IV. — On the Pressure Cavities in Topaz, Beryl, and Diamond, and their hearing 

 on Geological Theories. By Sir David Brewstee, K.H., F.R.S. 



(Read 3d March 1862.) 



In the years 1823 and 1826 I communicated to this Society two papers 

 " On the Existence of Two New Fluids in the Cavities of Precious Stones and 

 other Minerals." These two fluids were generally found together in the same 

 cavity, though sometimes the cavities were occupied only by one of them. They 

 were perfectly transparent and immiscible. The denser of the two occupied 

 the angles of the cavities, or the necks, or narrow passages, or canals which 

 united two or more larger cavities ; while the rarer fluid floated, as it were, on 

 the other in deep cavities, or filled the body of shallower ones, with the exception 

 of a circular vacuity, which diminished and disappeared with the slightest increase 

 of temperature, or enlarged itself and disappeared in consequence of the fluid 

 being converted into vapour. 



The denser of these fluids does not appear to expand more than oil or water 

 by the application of heat ; but the other is twenty-one times more expansible than 

 water. It evaporates at temperatures from 74° to 84". The vacuity in it disap- 

 pears by the heat of the mouth or of the hand, and it returns to its former state 

 by a violent effervescence, producing a number of minute vacuities, which finally 

 unite in one. The refractive power of the expansible fluid varies from 1-1311 to 

 1*2106, while that of the denser fluid is 1-2946, which is greatly less than that 

 of water. From the few experiments which I was able to make on these fluids 

 when taken out of the cavities, it has been inferred that they are hydro-carbons. 



The distribution of these cavities, in the specimens which contain them, is a 

 subject of peculiar interest. They are often found singly, and of different sizes, 

 at different depths in the mineral ; but they most frequently occur in strata, and 

 of such different magnitudes, that the two fluids are distinctly seen in the largest, 

 while the rest gradually diminish till they disappear in black points, which the 

 microscope can hardly descry. Three or four strata nearly parallel to one an- 

 other, and with cavities of different sizes, rarely occur. In general the strata lie 

 in planes, frequently intersecting one another, and having no connection with the 

 primitive or secondary planes of the crystal. In some specimens the planes of 

 the strata are curved, and in rare cases the sections of these planes are curves 

 of contrary flexure. 



In 1844 I was led to re-examine several hundred specimens of topaz with a 

 more perfect microscope and a fine polarising apparatus, with the view of ascer- 



* The American and French mineralogists have given the name oi Brewstolinc to the volatile, and 

 Cryptoline to the dense fluid. 



VOL. XXIII. PART I. L 



