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 A E. Mj^ attention having been thus directed to this part 

 of the specimen, I Avas surprised to observe a great number of capillary lines or 

 pipes P Q,, rising from the edge A E of the cavity, and into Avhich the fluid was 

 forcing itself, oscillating in these minute tubes like the mercury in a barometer, 

 and sometimes splitting the laminae 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, Avhen in tlie 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 mno, all the rest of the 

 cavity being much shallower. There was a fine doubly refracting crj^stal at M N, 

 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 to a 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 Avere, 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 mai-gin 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, Avhich 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 



