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position, and have every possible curvature ; their shape is equally 

 irregular ; and it is probable that, in every case, some edge or angle 

 of the stratum touches the surface. The cavities are sometimes con- 

 centrically arranged, and sometimes occur in parallel straight lines. 

 In one specimen, they radiate from a centre. When different strata 

 occur in the same specimen, they generally differ in the character of 

 the cavities, one stratum containing flat, another deep cavities, &c. 

 The whole facts lead to the conclusion, that the strata of cavities 

 have been formed under the influence of forces propagated through 

 a plastic mass, carrying with them gases and vapours, which came 

 to a position of rest previous to the crystallization of the mineral. 

 In Section II. the author describes some new observations on the 

 two fluids formerly discovered by him. In some cases of flat 

 cavities, the faces of which are parallel to the planes of easy clea- 

 vage, the application of heat forced the fluid between the laminae of 

 the crystal to a distant part. In one specimen, a white ball was 

 seen to be projected from one cavity to the edge of the specimen, as 

 in a case formerly described. In other specimens, where the dense 

 fluid was accompanied by a bubble of some gas, the application of 

 heat increased the size of the bubble, which then threw off a smaller 

 to a distant spot. On cooling, the latter disappeared, and the former 

 recovei-ed its original size. Apparently the g'as was here absorbed 

 by the liquid on cooling. While the bubble expanded by heat, the 

 liquid was forced into minute tubes or slits, from which, on cooling, 

 it returned. In Section III. he described the form and position of 

 the crystals in the cavities of topaz. They are both fixed and 

 moveable, and often beautifully formed. They are very numerous, 

 and occur in several different forms, which are enumerated, a very 

 frequent one being the cube. In Section IV. he treated of the 

 physical properties of tliese crystals. Many of them melt or dissolve 

 in the fluid in a gentle heat, others with difficulty, some not at all. 

 Those which melt are commonly I'eproduced, on cooling, of their 

 original form, with modifications. The tessular crystals have no 

 action on polarized light; but there seem to be two substances in 

 this form, as some tessular crystals melted, while others were found 

 infusible in any heat used. The doubly refracting crystals would 

 appear to be of three kinds, as some melted easily, others with dif- 

 ficulty, others not at all. They did not depolarize white light, or 

 the highest order of colours. One crystal melted, and was repro- 

 duced, without any fluid being present. In another cavity, several 

 crystals, when heated, darted across the cavity, while others rotated 



