in the Cavities of Minerals. • 499 



which the cavities lie in concentric arches, and have their sides 

 concentric, and, as it were, a portion of the same arches, as if 

 they had been formed under the influence of a rotatory force. 

 In other cases they occupy parallel lines, and are sometimes 

 so equidistant that they might be advantageously used as mi- 

 crometers for microscopes. In one remarkable specimen they 

 radiate from a centre, each radiation having a character of its 

 own. One radiation will -sometimes throw off a diverging 

 branch, while two or more radiations will converge and then 

 diverge again, subsequently uniting themselves into a single 

 radiation. 



When different strata of cavities lie parallel to each other 

 in the specimen, which they sometimes do, to the number of 

 four ovjivey each stratum has generally a distinct character ; 

 flat and exceedingly thin cavities occupying one stratum, very 

 deep cavities occupying another, minute cavities which the 

 highest magnifying powers can scarcely resolve occupying a 

 third, while a fourth consists of the most irregular and inde- 

 scribable forms. 



When the forms of individual cavities are related to that of 

 the stratum which contains them, they, of course, cut at all 

 angles the primary and secondary planes of crystallization ; 

 and the same is true of insulated cavities of great length, which 

 are sometimes turned, and twisted, and bent in the most ca- 

 pricious manner. It is impossible to read these details, and 

 still more so to study the phaenomena themselves, without 

 being driven to the conclusion, that the strata of cavities must 

 have been formed under the influence of forces propagated 

 through a soft and plastic mass, and carrying along with them 

 gases and vapours which came to a position of rest previous 

 to the regular crystallization of the topaz. This conclusion, 

 which I have been led to draw, in another paper, from a series 

 of entirely different facts, will be still further confirmed by 

 the phaenomena of imbedded crystals, to which I shall have 

 to refer in another section. 



2. Additional Observations on the Nature and Properties oj 

 the two New Fluids. 



In re-examining the phaenomena exhibited by the two nev^ 

 fluids, I have found no occasion to modify or to correct any 

 of the results contained in my former papers. In the cavities 

 which appear to contain only one fluid, namely, the dense 

 fluid, I have sometimes found a very small quantity of the 

 volatile fluid, which, with a slight rise of temperature, passes 

 into vapour, and prevents the apparent vacuity from disap- 

 pearing by the application of a strong heat. When there is 



2 K2 



