IN THE CAVITIES OF MINERALS. 15 
otherwise a very perplexing one. Having applied too strong a heat to the speci- 
men, the bubble A threw off beside B two or three smaller ones, which moved 
along the upper edge AE. My attention having been thus directed to this part 
of the specimen, I was surprised to observe a great number of capillary lines or 
pipes P Q, rising from the edge A E of the cavity, and into which the fluid was 
forcing itself, oscillating in these minute tubes like the mercury in a barometer, 
and sometimes splitting the laminz between them. The force of cohesion, thus 
overcome by the expansive efforts of the fluid, predominated over the capillary 
attraction of the tubes and surfaces, and pressed back all the fluid into the cavity, 
when the body of fluid had contracted in cooling. 
If we now consider the body which occupies the vacuity A as a gas, and, con- 
sequently, the other bubble B as the same, it follows, that the whole of the gas 
in B was absorbed by the fluid while cooling, and again given out by an increase 
of temperature. The gas, when in the act of being discharged, took its course to 
the locality of the speck at B, and to the bubble A ; but to the bubble A alone when 
the speck had disappeared. 
Upon repeating these observations the cavity burst; and I have now before 
me its two halves, forming its upper and its under surface. The portion of the 
cavity at A has the same depth as the portion below m7 0, all the rest of the 
cavity being much shallower. There was a fine doubly refracting crystal at MN, 
which polarised the blue of the second order; and its outline is still left on the 
cavity. There was a sort of crystalline powder disseminated round M N toa con- 
siderable distance, and the roof of the bubble B, when the roof of the cavity was 
entire, was always mottled with this powder. 
In a former paper, I have distinguished vapour cavities from common cavities, 
by the manner in which the vacuity in the expansible fluid disappears. In the 
one case, the vacuity gradually enlarges by the degradation, as it were, of its mar- 
gin, as the fluid passes into vapour ; in the other, the vacuity gradually diminishes 
till it disappears. I have since found cavities of an intermediate character, in 
which the vacuity, on the first application of heat, diminishes, and then, when it 
has contracted to a certain size, it begins to expand; and its margin becoming 
thinner and thinner, it finally passes into vapour. 
3. On the Form and Position of Crystals in the Cavities of Topaz. 
In a former paper I have described a moveable group of crystals of carbonate 
of lime, which I discovered in a cavity in quartz from Quebec, containing a fluid 
with the properties of water. The crystals to which I am about to call atten- 
tion, are of a very different kind, and possess a very different kind of interest. 
The crystals which occupy the fluid cavities of topaz are either fixed or 
