42 SIR DAVID BREWSTER ON THE 



Having found in diamond so many Pressure Cavities^ as we may call them, 

 round which the substance of the stone is compressed, I had some expectation of 

 finding- them in other minerals ; and upon re-examining the numerous plates of 

 topaz in my possession, I succeeded in discovering several under such remark- 

 able circumstances, that I submitted a description and drawings of them to this 

 Society in 1845. In searching for this phenomenon with the polarising micro- 

 scope, we first observe four sectors of depolarised light ; and if the magnifying 

 power is sufficient, we shall find, in the centre of the black cross that separates 

 the sectors, a small opaque speck, which is the cavity or seat of the compressing 

 force. This cavity is frequently of a rhomboidal form, and often only the 3000th 

 or 4000th of an inch in diameter. It is always opaque, as if the elastic substance 

 which 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 polarised 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 gene- 

 rally of a larger size, in which the 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 com- 

 pression, 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 Tweed- 

 dale, who was so kind as to present it to me. In cutting the cr^^stal, Mr Sander- 

 son 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 specimens 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 con- 

 tents 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 luminous sectors produced a mass of depolarised 

 light, which completely 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 mine- 

 rals, 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 



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



