in the Cavities of Minerals. 269 
the water as nearly pure *<, I was, therefore, in no small degree 
surprised, when I discovered, in a cavity of a quartz-crystal from 
Quebec, from the cabinet of Mr Allan, not only insulated crys- 
tals, but a tolerably large group, which were moveable through 
the fluid upon turning round the specimen^. The crystal was 
perfectly sound around the cavity, which had a sort of triangu- 
lar form, one of the sides of the triangle being about one-tenth 
of an inch long. The fluid was quite transparent ; and, as the 
air-bubble was not perceptibly diminished by heat, there is every 
reason to think that the fluid is water. The crystals were trans- 
parent to a considerable degree, and had a white milky tint, 
when seen by reflected light. 
In considering the circumstances of this singular phenome- 
non, we are led to suppose, that the included crystals had been 
dissolved in the fluid at the time of its being shut up in the 
quartz, and had afterwards been deposited from the solution. 
The ingenious supposition of Sir Humphry Davy, that a liquid 
hydrate of silica may exist at high temperatures, and may con- 
tain small quantities of atmospheric air, will no doubt explain 
the phenomena of water in rock-crystals ; but it is not easy to 
comprehend how the formation of a group of crystals could either 
have accompanied or followed the separation of the water and 
the silex. 
As the specimen now alluded to is too valuable to be destroyed, 
for the purpose of analysing the minute crystals, it is probable, 
that our information respecting them would have been very li- 
mited, had not a circumstance of an accidental nature enabled 
me to throw some farther light on the subject. Several years 
ago, when I was examining, along with Earl Compton, a large 
collection of quartz-crystals from Quebec, for the purpose of ob- 
taining remarkable crystallizations, I was much struck with the 
appearance of several spherical groups of whitish crystals, with- 
in some of the specimens. Upon pointing out to Lord Comp- 
ton this peculiarity, his Lordship agreed with me in thinking 
* In another part of his interesting paper, Sir Humphry Davy makes the fol- 
lowing observation: « The fluid, in all the crystals, (in two it was minutely exa- 
mined) was found to be water, nearly pure, containing only a minute portion of the 
alkaline sulphates.” Phil. Trans . 1822, p. 370.— Ed. 
t There were also numerous opaque particles in the cavity, which descended 
slowly in the fluid- 
