MODIFIED by COMPRESSION. 129 



fhell had fhrunk upon itfelf, fo as to ftand detached from 

 the fides, and bore very ftrong marks of fufion. The exter- 

 nal furface was quite fmooth, and mining like an enamel. 

 The internal part confifted of a mixture of large bubbles 

 and folid parts : the inlide of the bubbles had a luflre much 

 fuperior to that of the outfide, and equal to that of glafs. The 

 general mafs was femitranfparent ; but fmall parts were vifible 

 by the lens, which were completely tranfparent and colour- 

 lefs. In feveral places this fmooth furface had cryflallized, 

 fo as to preient brilliant facettes, fleadily mining in certain 

 afpects. I obferved one of thefe facettes on the infide of an 

 air-bubble, in which it interrupted the fpherical form as if 

 the little fphere had been prefTed inwards at that fpot, by the 

 contact of a plane furface. In fome chalk near the mouth of 

 the large tube, which lay upon a flratum of filex, another very 

 interefting circumflance occurred. Connected with its lower 

 end, a fubflance was vifible, which had undoubtedly refulted 

 from the union of the carbonate with the lilex. This fubflance 

 was white and femitranfparent, and bore the appearance of 

 chalcedony. The mafs of chalk having attached itfelf to that 

 above it, had fhrunk upwards, leaving an interval between it 

 and the filex, and carrying fome of the compound up with it. 

 From thence this laft had been in the act of dropping in a 

 vifcid ftate of fufion, as evidently appeared when the fpecU 

 men was entire 3 having a flalactite and ftalagmite corre- 

 fponding accurately to each other. Unluckily I broke off the 

 flalactite, but the ftalagmite continues entire, in the form of a 

 little cone. This new fubflance erfervefced in acid, but not 

 brifldy. I watched its entire folution ; a fet of light clouds 

 remained undiffolved, and probably fome jelly was formed • 

 for I obferved, that a feries of air-bubbles remained in the 

 form of the fragment, and moved together without any vifible 

 connection ; thus feeming to indicate a chemical union be- 

 Vol.VI.— P. I. R twecn 



