■'■ • »V ■■c<3MPRE^-^iO'*r. SB7 



rircumihnce may hereafter throw h'ght on the geological 

 hiftory of this extraordinary fubftance, 



. I have faid, that, by mechanical conftraint, almofl the And moft of tU« 



^ 1 • I carbonic acid 



whole of the carbonic acid was retained. And, in truth, at ^as retained. 



this period, fome lofs of weight had been experienced in all 



the experiments, both with iron and porcelain. But even this 



pircumftance is valuable, by exhibiting the influence of the 



carbonic acid, as varied by its quantity. 



When the lofs exceeded JO or 15 per cent. * of the weight Qualities of the 

 «f the carbonate, the refult was always of a friable texture, f^°ccordingt(» 

 and without any ftony chara6ler ; when lefs than two or three the lofs of cat-, 

 percent, it wasconfidered as good, and poflelFed the properties ^^^ ^* 

 of a natural carbonate. In the intermediate cafes, when the 

 lots amounted, for inftance, to fix or eight per cent, the re- 

 fult was foraetimes excellent at firfl, the fubfiance bearing 

 every appearance of foundnefs, and often pofl'elTing a high cha- 

 rafter of cryfiallization ; but it was unable to refift the ac- 

 tion of the air; and, by attrafting carbonic acid or moifiure, 

 or both, crumbled to diifl more or lefs rapidly, according to 

 circumflances. This feems to prove, that the carbonate of 

 lime, though not fully faturated with carbonic acid, may pof- 

 fefs the properties of limefione; and perhaps a difference of 

 this kind may exift among natural carbonates, and give fife ta 

 their different degrees of durability. 



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

 reached only to a certain depth into t!ie 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 frnall 

 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 i^ 

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

 time, and the other of a firor.g cuute, afting for a fhort 

 time. 



Some of the refuUs whicli' feemed the moft perfe6l when Some refufe 

 firft produced, have been fubiect to decav, owing to partial ^'^""^ ^".^j^'^ " 



' . " or decay trota 



calcination. It happened, in fome degree, to. the beautiful partial calcina- 



tion. 

 * I have found, that. In open fire, the cutiie lofs fuftained by the 



carbonate varies in difFei ent kir,d<. from 4i? t« M.'o per cent. ■■: 



fpecimen 



