56 
Emerald. 
Some of the specimens which we have examined are so full of fluid- 
cavities that they are only partially transparent. They differ entiiely 
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 Transactions of the Geological ^Societyf : 
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 the fact. 
The form is clearly that of a crystal, and it depolarizes light very power- 
fully. 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. 
* " Qeber Eiuschliisse im Diamant," Naturkundige Verhandlungen, 
Haarlem, 1864. 
f 2ad series, vol. iii # p. 455. 
