82 
^are quartz (rock crystal, chalcedony, &c.), topaz, emerald, 
&c. In some specimens of quartz, the milky appearance is 
entirely due to myriads of minute cavities. 
The contained fluid usually consists of either water or 
an aqueous solution of salts — commonly chloride of sodium. 
In this latter case crystals of chloride of sodium may often 
he seen in the fluid. Besides the above, solution of carbonic 
acid is met with, and even pure liquid carbonic acid. 
No. i is a specimen of chalcedony from Uraguay, and is 
chiefly remarkable for the quantity of fluid contained, it 
being about 60 grains. On shaking, the fluid may be heard 
to rattle, and the mineral being transparent the bubble 
may easily be seen on holding the specimen to the light. 
Another peculiarity is the absence of any indications of 
attachment to any other body on the surface of this mineral, 
rendering its mode of formation a matter of doubt. 
No. ^ is a crystal of quartz from Brazil. Three cavities 
may be seen ; one about one-eighth of an inch in diameter 
contains a viscid fluid, together with a vacuole arid an 
irregular black fragment of mineral matter. Both these 
are movable when the specimen is rotated. One of the 
other cavities contains fluid and a portion of rock magma. 
In neither of these specimens (1 and 2) can the vacuoles 
be made to disappear by heat. 
No. 3. A piece of rock-crystal containing numerous flat- 
tened irregular cavities. Each is seen to contain a flattened 
vacuole, surrounded by fluid, which fluid is separated from 
the walls of the cavities by a space which, according to 
Brewster, contains a second fluid of different optical density. 
On heating, the vacuole is seen to decrease in size, and 
Anally to disappear. The contour line of the second fluid 
remains unchanged. On cooling, the vacuole reappears. 
No. Section of quartz containing fluid cavities, each 
containing a crystal of common salt and a vacuole. The 
vacuoles are motionless and do not disappear on heating. 
