178 Sir David Brewster on the Pressure Cavities 



it contained had collapsed into a black powder; and I have met 

 with only one cavity in which there was a speck of light in its 

 centre. The polarized tint in the luminous sectors varies from 

 the faintest blue to the white of the first order. In most cases 

 the elastic force has spent itself in the compression of the topaz, 

 the cavity remaining entire, and without any apparent fissure by 

 which a gas or a fluid could escape. I have discovered, however, 

 other cavities, and these generally of a larger size, in which th^e 

 sides have been rent by the elastic force, and fissures, from one 

 to six in number, propagated to a small distance around them. 

 These fissures have modified the doubly refracting structure 

 produced by compression, but the gas or fluid which has 

 escaped has left no solid matter on the faces of fracture. 



Soon after the publication of these results, I discovered still 

 more remarkable cavities in a specimen of beryl brought from 

 India by the Marchioness of Tweeddale, who was so kind as to 

 present it to me. In cutting the crystal, Mr. Sanderson found 

 that one end of it was foul, and produced a luminous ring round 

 a candle. This ring, similar to the rings seen in certain speci- 

 mens of Iceland spar, was produced by long and irregularly 

 tubular cavities parallel to the sides of the hexagonal prism. 

 As the tubes had been cut across by the lapidary, their contents 

 had escaped ; but whatever the contents were, whether fluid or 

 gaseous, they had compressed the beryl, and produced the four 

 luminous sectors around each cavity. This aggregation of lumi- 

 nous sectors produced a mass of depolarized light, which com- 

 pletely effaced the black cross of the uniaxal system of rings 

 exhibited by the mineral. Different degrees of compression 

 were produced by cavities of different sizes ; but the resulting 

 tint was generally a white of the first order, rising in some cases 

 to a yellow of the same order. 



Such is a brief notice of the fluid- and pressure cavities which 

 exist in minerals, and which have a very obvious bearing on 

 geological theories. Some of these facts have been upwards of 

 forty years before the public*, and, along with others more 

 recently discovered, have been widely circulated in British and 

 foreign journals; and yet none of our geologists have made the 

 slightest reference to them, either as difficulties to be explained, 

 or arguments to be advanced in support of their own views. 



In 1822 Sir H. Davy, when he was acquainted only with the 

 existence in minerals of water, petroleum, and gas, did not hesi- 

 tate to regard such facts as "seeming to afford a decisive argu- 

 ment in favour of the igneous theory of crystalline rocks "f; 

 and in my paper of 1826 I was driven to the conclusion "that 



* Edinburgh Philosophical Journal, vol. ii. p. 334, 1820. 

 t Philosophical Transactions, 1822, p. 367. 



