1869.] Rubies, Sapphires, Diamonds, and some other Minerals. 295 



vapour, as occurred in some of the cavities in topaz described b}*- Brewster. 

 In this change it must expand about six hundred times less than when 

 water passes into steam. Spinel also incloses crystals of several other 

 minerals which we have not yet been able to identify. 



Aquamarina. 



The most striking peculiarity of this mineral is the occurrence of num- 

 bers of fluid-cavities containing two fluids and a vacuity, as shown bv 

 fig. 6. 



Emerald. 



Some of the specimens which we have examined are so full of fluid - 

 cavities that they are only partially transparent. They differ entirely 

 from those already described, and contain only one liquid, which does not 

 sensibly expand when warmed. In all probability this is a strong saline 

 aqueous solution, since the cavities also inclose cubic crystals, as shown 

 by fig. 8, which dissolve on the application of heat, and recrystallize on 

 cooling. On the whole, therefore, these cavities are very similar to those 

 found in the quartz of some granites, and in some of the minerals found in 

 blocks ejected from Vesuvius, as described in Mr. Sorby's paper on the 

 microscopical structure of crystals, already referred to. 



Diamond* 



Few, if any, of the specimens of diamond that have come under our notice 

 contain objects similar to those which, in the opinion of Goppert*, are evidence 

 of its having been derived from vegetable remains, but we have been able to 

 study to great advantage some facts which do not appear to have presented 

 themselves to either Goppert or Brewster. We have examined twenty-one 

 objects similar to the two described by Brewster, in his paper in the Trans- 

 actions of the Geological Society t; and this has enabled us to clear up some 

 of the difficulties to which he alludes, and has led us to propose a different 

 explanation. He thought that the black specks, which were surrounded 

 by a black cross when examined with polarized light, were minute cavities ; 

 but at the same time he admitted that they were so small that it was not 

 possible to say whether they contained a fluid or were empty. Judging from 

 what we have seen of such small examples, we consider it impossible to say 

 whether they are cavities or inclosed crystals ; but fortunately we have 

 met with several of such a size and character that it was quite easy to see 

 that they were crystals. Fig. 9 is a most excellent example of this fact. 

 The form is clearly that of a crystal, and it depolarizes light very powerfully. 

 Its refractive power must be very much less than that of diamond ; for the 

 inclined planes totally reflect the transmitted light, and thus look quite 

 black, as shown in the figure. It is this circumstance which causes many 

 smaller inclosed crystals to appear like mere black specks. 



* " Ueber Einscnliisse im Diamant," Natuurkundige Yerkandelingen, Haarlem, 

 1864. t 2nd series, vol. iii. p. 455. 



