BY COMPRES^rO"N. 395 



feveral places, particularly at the top, the carbonate was very 

 porous, and full of decided air-holes, which could not have 

 been formed but in a foft fubftance ; the globular form and 

 ftiining furface of all thefe cavities, clearly indicating fuOon. 

 The fubftance was femitranfparent ; in fome places yellow, 

 and in fome colourlefs. When broken, the folid parts (hewed 

 a faline fra6ture, compofed of innumerable faceltes. The 

 carbonate adhered, from end to end, to tlie tube, and in- 

 corporated with it, fo as to render it impoffible to afcertain 

 what lofs had been fuftained. In general, the lineofconlaa was 

 of a brown colour; yet there was no room for ("ufpeding the 

 prefence of any foreign matter, except, perhaps, from the iron- 

 fod which was ufed in ramming down the chalk. But, in fub- 

 fequent experiments, I haveobferved the fame brown or black 

 cdouf at the union of the carbonate with the porcelain tubes, 

 where the powder had been purpofely rammed with a piece of 

 wood ; fo that this colour, wliich has occurred in almoft every 

 fimilar cafe, remains to be accounted for. The carbonate ef- 

 fervefced violently with acid ; the fubftance in contad with the 

 tube, doing fo, however, more feebly than in the heart, leav- 

 ing a copious depcfit of while fandy matter, which is doubtlefs 

 a part of the tube, taken up by the carbonate in fufion. 



On the 2-'tih of March. I made a fimilar experiment, in a Another ck- 

 ftout gun-barrel, and took fome care, after the application f^^H'^^^^y^'^^ 

 heat, 10 coo! the barrel flowly, with a view to cryflallization. Saline ftruSure 

 The whole mafs was found in a fine ftate, and untouched by ^^^^^^'^^^^^'J'^^^^T^ 

 the lead; having a feraitranfpareiU and faline flru6lure, with chalk previoufly 

 various facettes. In one part, I found the moft decided cry- P''""'^^^' 

 flallization I had obtained, though of a fmall fize: owing to 

 its tranfparency it was not eafily vifible, till the light was made 

 to reflect from the cryftalHne furface, which then produced 

 a dazzle, very obfervable by the naked eye ; when examined 

 by means of a lens, it was feen to be compofed of feveral 

 plates, broken irregularly in the fra6lure of the fpecimen, all 

 of which are parallel to each other, and refled under the fame 

 angle, fo as to unite in producing the dazzle. This flrudure 

 was obfervable equally well in both parts of the broken fpeci- 

 men. In a former experiment, as large a facette was obtain- 

 ed in a piece of folid chalk; but this refult was of morecon- 

 fequence, as having been produced from chalk previoudy 

 pounded. 



' The 



