MODIFIED ly COMPRESSION. 97 
this kind may exift among natural carbonates, give rife io 
their different degrees of durability. 
I nave 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 {mall 
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 effec 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 tefults which feemed the moft perfect when 
firft produced, have been fubje& 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 
1sth May 1801, 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 amarket. Yet, ina 
few weeks after its formation, it fell to duft. 
Numser-ess fpecimens, however, have been obtained, which 
refit the air, and retain their polifh as well as any marble. 
Some of them continue in a perfec 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. § 
Vot. VI.—P. I, N A 
