Properties of the two New Fluids in Minerals. 427 



paper. The singular fact, however, is, that the portion no of the 

 cavity quitted by the globule of air, was immediately filled up 

 with ice, and the cavity reduced to the dimensions m n. 



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 *. 



In concluding tnese observations, I could have wished to en- 

 ter into some details respecting their geological relations ; but as 

 these would lead us too far into the regions of speculation I shall 

 not enter upon them on the present occasion. It may be proper, 

 however, to state, that the opinion which I hazarded in a former 

 paper, that the discovery of the two New Fluids in minerals at- 

 tached a new difficulty to the aqueous hypothesis, has been ren- 

 dered more probable by every subsequent inquiry ; and that I can 

 see no way of accounting for the phenomena, but by supposing 

 that the cavities were formed by highly elastic substances, when 

 the mineral itself had been either in a state of fusion, or ren- 

 dered soft by heat. 



* Since this Paper was written, Mr WILLIAM NICOL has shewn me a very re- 

 markable specimen of Sulphate of Barytes, with fluid cavities of the same general 

 character with those which I described in my former paper (Trans, vol. x. p. 36.), 

 but much larger than any which I had seen. Upon grinding down on a dry stone, 

 one of the faces of this specimen, the largest cavity burst, and discharged its fluid 

 contents through the fissure upon the ground surface of the specimen. The fluid 

 lay in drops of different sizes along the line of the fissure, and in this condition Mr 

 NICOL put it into his cabinet. Upon looking at the specimen about twenty-four 

 hours afterwards, each drop of fluid had become a crystal of Sulphate of Barytes. 

 These crystals had the primitive form of the mineral. 



This very curious fact is analogous to the uncrystallised water in the ice-cavities 

 mentioned above, the crystallisation in both cases being prevented by pressure. 

 When that pressure was removed, a portion of the water and the fluid sulphate of 

 barytes were immediately crystallised. Mr NICOL distinctly remarked, that the 

 crystals occupied as much space as the drops of the fluid ; so that the crystals of sul-, 

 phate of barytes were not deposited from an aqueous solution, but bore the same re, 

 lation to the fluid from which they were formed, as ice does to water, 



4 



