/. in the Cavities of Minerals. 35 



with heat. These crystals frequently burst with a heat not 

 above 150. In several cavities I have observed solid fragments 

 falling through the fluid, by the inversion of the crystal. 



9. SULPHATE OF LIME. In this mineral the cavities have of- 

 ten a very singular form ; and in all the specimens which I have 

 examined, the fluid is aqueous, and is accompanied with a gas or 

 a perfect vacuum. 



In Fig. 33. I have represented one of the most singular ar- 

 rangements of cavities that I have met with. In order to deter- 

 mine the thickness of the cavity, I reduced the specimen, so as 

 to give the polarized colours of the second order of Newton's 

 Scale, and, by carefully observing the difference of tint in the 

 cavities, and in the solid parts, I obtained a measure of the thick- 

 ness of crystalline matter abstracted in these parts. The differ- 

 ence of tint was very obvious, and proved that the thickness of 

 the cavity did not exceed the T oVo th part of an inch. 



In many specimens, these very shallow cavities occur in long 

 canals. In others they resemble some foreign crystalline matter, 

 shooting out into the most singular forms, as at a, b, Fig. 34. and 

 sometimes the cavities appear to the eye like tufts of white silk 

 compressed between the laminae, though they are, in reality, 

 strata of rhomboidal cavities, occurring in thousands, and ar- 

 ranged in the direction of their longest diagonals, while the stra- 

 ta themselves are highly inclined to the surfaces of the laminae. 

 In other specimens, the cavities have the most singular forms, as 

 represented in Fig. 34. One of the canals in sulphate of lime is 

 shewn at AB, in Fig. 35., where abed, efgk are two air- 

 bubbles or vacuities. By applying heat to the side B, these 

 air-bubbles shift their places. All the lines a b, cd, ef, gh, 

 advance to B, but c d and ef approach to one another, and the 

 moment they come in contact, the two vacuities are converted 

 into one, which has the position a' If g' h'. 



