418 DR BKEWSTEH on the Refractive Powers and other 



exhibited in a very remarkable manner, which I have described 

 and attempted to delineate in my former paper. Upon apply- 

 ing heat to a specimen of quartz, the elastic force of the impri- 

 soned fluid was such as to make it force its way through the so- 

 lid stone ; and when it had made its escape into the open air, 

 not a trace of its path was left behind. This phenomenon, which 

 was too extraordinary to present itself frequently, was afterwards 

 seen both by Mr SANDERSON and myself in a specimen of topaz, 

 when the fluid ascended through its substance with great rapidi- 

 ty, and resembled globules of quicksilver. This metallic ap- 

 pearance was owing to the total reflexion of light, which took 

 place at the refracting surface of the globule and the topaz. 

 That the fluid in this case forced its way through the clea- 

 vage planes of the mineral cannot be doubted, and I have in 

 another paper shewn, that fissures in glass may be closed up 

 without leaving the slightest trace of the two surfaces ever ha- 

 ving been separated *. 



In the various cavities described in my former paper, the 

 whole of the expansible fluid, when exposed to heat, was either 

 driven into vapour f , or retained its fluidity after it had filled 

 the vacuity. Since that paper was published, however, I have 

 discovered cavities in which, after the application of heat, there 

 may be said to be three different substances, viz. 1. The expansi- 

 ble fluid in a state of fluidity ; 2. The dense fluid ; and, 3. The 

 vapour of the expansible fluid. This curious fact will be under- 

 stood from Fig 5. of Plate XIX, which represents a cavity in a 

 specimen belonging to Mr SPADEN. The cavity is one-tivelfth of 

 an inch long. The expansible fluid is lodged at N N and N' N', 



* See Phil. Trans. 1816, p. 73. 



} One of the largest vapour cavities that I have seen is one-twelfth of an inch 

 every way. It is less than half full of fluid, and hence it is driven into vapour by 

 heat. During the precipitation of the vapour it becomes perfectly opaque. 



