186 



MR. J. B. DANCER ON 



origin, from a substance which had once been in a plastic 

 state, like amber and other gums. 



Prince Albert permitted Sir David to examine the Koh- 

 i-noor diamond ; this contained three black specks, scarcely 

 visible to the naked eye ; but under the microscope, with 

 polarized light, they were shown to be cavities, surrounded 

 by sectors of light. In the two smaller diamonds which 

 accompanied the Koh-i-noor there were several cavities 

 showing the same phenomenon. The examination of nearly 

 fifty diamonds lent by Messrs. Hunt and Roskell, showed 

 numerous cavities of the most singular forms, round which 

 the substance of the stone had been compressed. 



In some diamonds from the East-India Company^s 

 Museum, large cavities were found, which would have 

 prevented them from being cut into brilliants ; in fact, it 

 appears that diamonds are more subject to flaws than any 

 other stones used by jewellers. Some diamonds derive 

 their black colour from the number of cavities they con- 

 tain, which will not permit any light to pass through 

 them. 



About twenty-four years since, Sir David Brewster pre- 

 sented me with a specimen of topaz with fluid- cavities, the 

 first I possessed. Since that time I have been in the 

 habit of examining other crystals. Through the kindness 

 of friends I have had many interesting specimens for in- 

 spection ; amongst these, fluid-cavities have been found in 

 quartz crystals from South America, Norway, the Alps, 

 Ireland, Wales, and also in some quartz crystals which 

 Mr. E. W. Binney, F.B.S., brought from the Isle of Man. 



A specimen of fluor-spar, given to me by a friend, con- 

 tained a fluid- cavity much larger than any I had seen in 

 quartz and topaz. It was my intention to show this to 

 our Members ; but on trying some experiments for the 

 purpose of ascertaining the degree of heat required for 

 the expansion of the fluid, the cavity unfortunately burst ; 



