in Topaz, Beryl, and Diamond. 177 



diamond, and in other minerals supposed to be of aqueous forma- 

 tion, that its geological importance was likely to be acknowledged. 



In the Koh-i-noor diamond, which the Prince Consort kindly- 

 permitted me to examine in 1852, I found three black specks, 

 scarcely visible to the eye, but which the microscope showed to 

 be irregular cavities, surrounded with sectors of polarized light. 

 In the two smaller diamonds which accompanied the Koh-i-noor, 

 there were also several cavities surrounded with luminous sectors, 

 and the same polarizing structure, which indicated the operation of 

 compressing and dilating forces*. In order to obtain more 

 information on this subject, I examined nearly fifty diamonds 

 lent me by Messrs Hunt and Roskill, and in almost all of them 

 I found numbers of cavities, of the most singular forms, round 

 which the substance of the stone had been compressed and 

 altered in a remarkable manner. The shapes of the cavities 

 sometimes resembled those of insects and lobsters, and the 

 streaks and patches of colour in polarized light were of the most 

 variegated kind. In examining a large number of diamonds 

 which adorn some of the oriental objects in the East India 

 Company's Museum, I found that all these stones contained 

 large cavities, and were coarse or flawed diamonds, which could 

 not be cut into brilliants or used in rings or other ornaments. 

 It seems, indeed, to be a general truth that there are compara- 

 tively few diamonds without cavities and flaws, and that this 

 mineral is a fouler stone than any other used in jewellery. 

 Some diamonds, indeed, derive their black colour entirely from 

 the number of cavities which they contain, and which will not 

 permit any light to pass between them. 



Having found in diamond so many Pressure Cavities, as we 

 may call them, round which the substance of the stone is com- 

 pressed, I had some expectation of finding them in other mine- 

 rals ; and upon re-examining the numerous plates of topaz in 

 my possession, I succeeded in discovering several under such 

 remarkable circumstances that I submitted a description and 

 drawings of them to this Society in 1845f. In searching for 

 this phenomenon with the polarizing microscope, we first observe 

 four sectors of depolarized light ; and if the magnifying power is 

 sufficient, we shall find in the centre of the black cross that sepa- 

 rates the sectors a small opake speck, which is the cavity or seat 

 of the compressing force. This cavity is frequently of a rhom- 

 boidal form, and often only the 3000th or 4000th of an inch in 

 diameter. It is always opake, as if the elastic substance which 



* In 1820 I discovered similar cavities in amber, &c. See Edinb. Phil. 

 Journ. vol. ii. p. 334. 



f See Edinb. Trans. 1845, vol. viii. p. 15/ ; or Journal de Physique, 1846, 

 vol. lxxxii. p. 367. 



Phil. Mag. S. 4. Vol. 25. No. 167. March 1863. N 



