Properties of the Two New Fluids in Minerals. 135 



A very sharp frost occurred in Roxburghshire on the 

 morning of the 8th October 1825. The gravel-walks in the 

 garden were raised up about an inch above their natural level 

 by the sudden congelation of the water in the earth mixed 

 with gravel. All the elevated portions consisted of vertical 

 prismatic crystals of ice of six-sided prisms, with summits 

 which seemed to be triedral. The leaves of plants, &c were 

 covered with granular crystals, which were in general six-sided 

 tables. 



Upon examining, with a microscope, the prismatic crystals, 

 aggregated in parallel directions, they presented some curious 

 phenomena. They had numerous cavities of the most minute 

 kind, arranged in rows parallel to the axis of the crystals, and 

 at such equal distances as to resemble a series of mathemati- 

 cally equidistant points. Some of the cavities were very long 

 and flat, and sometimes they were amorphous ; but, in general, 

 they contained water and air. 



Upon submitting one of these cavities to a powerful micro- 

 scope, it appeared, as shown in Fig. 1 4, where A B C is the 

 piece of ice, having in it a long cavity m o, containing water 

 and air. The ice gradually dissolved ; and when the end n o 

 of the cavity mn was near the edge of the ice C B, the air, in 

 a portion of it n o, detached itself, and went off atj9, through 

 the solid ice, the cavity closing up again at n. This phe- 

 nomenon is analogous to the passage of the expansible fluid 

 through topaz and quartz, which has been already described ; 

 the air in the one case, and the fluid in the other, finding its 

 way in the direction of easiest cleavage, and the fissure closing 

 up again in the manner already mentioned in a preceding part 

 of this paper. The singular fact, however, is, that the por- 

 tion n o of the cavity quitted by the globule of air, was imme- 

 diately filled up with ice, and the cavity reduced to the di- 

 mensions mn. 



As the formation of ice from water is in every respect analo- 

 gous to the formation of crystals, from a substance rendered 

 fluid by heat, the examination of its cavities is likely to throw 

 some light upon their formation in mineral bodies.* 



* Since this paper was written, Mr William Nicol, Lecturer on Natu- 

 ral Philosophy, has shown me a very rem irkable specimen of Sulphate of 



