MODIFIED by COMPRESSION. 97 



this kind may exift among natural carbonates, give rife to 

 their different degrees of durability. 



I have obferved, in many cafes, that the calcination has 

 reached only to a certain depth into the mafs ', the internal 

 part remaining in a ftate of complete carbonate, and, in ge- 

 neral, of a very fine quality. The partial calcination feems 

 thus to take place in two different modes. By one, a fmall 

 proportion of carbonic acid is taken from each particle of 

 carbonate ; by the other, a portion of the carbonate is quite 

 calcined, while the reft is left entire. Perhaps one refult is 

 the effect of a feeble calcining caufe, acting during a long 

 time, and the other of a ftrong caufe, acting for a fhort time. 



Some of the refults which feemed the moft perfect when 

 firft produced, have been fubject to decay, owing to partial 

 calcination. It happened, in fome degree, to the beautiful 

 fpecimen produced on the 3d of March 1801, though a freih 

 fracture has reftored it. 



A specimen, too, of marble, formed from pounded fpar, on 

 15th May 1 801, was fo complete as to deceive the workman 

 employed to polifh it, who declared, that, were the fubftance 

 a little whiter, the quarry from which it was taken would be 

 of great value, if it lay within reach of a market. Yet, in a 

 few weeks after its formation, it fell to duft. 



Numberless fpecimens, however, have been obtained, which 

 refifl the air, and retain their polifh as well as any marble* 

 Some of them continue in a perfect ftate, though they have 

 been kept without any precaution during four or five years. 

 That fet, in particular, remain perfectly entire, which were 

 fhewn laft year in this Society, though fome of them were 

 made in 1799, fome in 1801 and 1802, and though the firft 

 eleven were long foaked in water, in the trials made of their 

 fpecific gravity. 



Vol. VI P.I, N A 



