126 EFFECT'S of HEA? 



I found, that chalk expofed to a heat above that of boiling 

 water, but quite fhort of rednefs, loft 0.34 per cent. ; and in 

 another fimilar trial, 0.46 per cent. Now, this lofs of weight 

 equals within 0.01 per cent, the lofs in the 1 aft-mentioned ex- 

 periment, that being 0.47 ; and far furpafles that of the laft 

 but one, which was but 0.074. There is good reafon, there- 

 fore, to believe, that had the carbonate, in thefe two laft ex- 

 periments, been previoufly dried, it would have been found 

 during compreilion to have undergone no lofs. 



The refult of many of the experiments lately mentioned, 

 feems fully to explain the perplexing difcordance between my 

 experiments with porcelain tubes, and thofe made in barrels of 

 iron. With the procelain tubes, I never could fucceed in a 

 heat above 28°, or even quite up to it; yet the refults were 

 often excellent. Whereas, the iron-barrels have currently 

 ftood firm in heats of 41 ° or 51 , and have reached even to 

 70 or 80 ° without injury. At the fame time, the refults, 

 even in thofe high heats, were often inferior, in point of fu- 

 fion, to thofe obtained by low heats in porcelain. The rea- 

 fon of this now plainly appears. In the iron-barrels it has 

 always been confidered as necefTary to ufe an air-tube, in con- 

 fequence of which, fome of the carbonic acid has been fe- 

 parated from the earthy bails by internal calcination : what 

 carbonic acid remained, has been more forcibly attracted, ac- 

 cording to M. Berthollet's principle, and, of courfe, more 

 eafily comprefTed, than when of quantity fufficient to faturate 

 the lime : but, owing to the diminiflied quantity of the acid, 

 the compound has become lefs fufible than in the natural 

 ftate, and, of courfe, has undergone a higher heat with lefs 

 efted. The introduction of water, by furniihing a reading 

 force, has produced a ftate of things fimilar to that in the 

 porcelain tubes ; the carbonate fuftaining little or no lofs of 



weight, 



