174 Sir David Brewster on the Pressure Cavities 



power, since its molecular condition in common temperatures 

 renders it alone capable to take that position, is an argument in 

 favour of its adoption also as unit of resistance. It is, indeed, 

 in practice almost a necessity to define the common resistance 

 unit as the resistance of a body of the same material as is chosen 

 for unit of conductibility, by which means calculations with it 

 are facilitated, and a perfect notion of the measure secured. 



An absolute unit might be adopted as measure of electro- 

 motive force even were a defined body of mercury to hold the 

 place of resistance unit : it must, however, be confessed that the 

 system of measurement would lose in consistency and con- 

 formity to the purpose; besides which the determination of 

 electromotive force in absolute measure is burthened with 

 material sources of error, and is therefore opposed by the same 

 objections which are tenable against the absolute resistance unit. 



Dr. Siemens has lately given his attention to the construction 

 of a reproducible unit of electromotive force, capable of being 

 exactly defined, and commodious, and will shortly publish the 

 results of his endeavours. 



XXII. On the Pressure Cavities in Topaz, Beryl, and Diamond, 

 and their bearing on Geological Theories. By Sir David 

 Brewster, K.H., F.R.S.* 



IN the years 1823 and 1826 1 communicated to this|Society two 

 papers " On the Existence of Two New Fluids in the Cavi- 

 ties of Precious Stones and other Minerals." These two fluids f 

 Were generally found together in the same cavity, though some- 

 times the cavities were occupied by only 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 dis- 

 appeared 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 disappears 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 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiii. 

 part 1. Communicated by the Author. 



t The American and French mineralogists have given the name of Brew- 

 stoline to the volatile, and Cryptoline to the dense fluid. 



