MODIFIED by COMPRESSION. 99 



a greater or a lefs degree, in all thefe experiments. The que- 

 ftion naturally fuggefts itfelf, What has become of the car- 

 bonic acid, feparated in thefe partial calcinations from the 

 earthy bans ? Has it penetrated the veffel, and efcaped en- 

 tirely, or has it been retained within it in a gafeous, but 

 highly comprefTed ftate ? It occurred to me, that this que- 

 ftion might be ealily refolved, by weighing the veffel before 

 and after the action of heat upon the carbonate. 



With iron, a conftant and inappreciable fource of irregu- 

 larity exifted in the oxidation of the barrel. But with porce- 

 lain the thing was eafy ; and I put it in practice in all my ex* 

 periments with this material, which were made after the que- 

 ftion had occurred to me. The tube was weighed as foon as its 

 muzzle was clofed, and again, after the breech had been expo- 

 fed to the fire; taking care, in both cafes, to allow all to cool. 

 In every cafe, I found fome lofs of weight, proving, that even 

 in the beft experiments, the tubes were penetrated to a certain 

 degree. I next wifhed to try if any of the carbonic acid fepara- 

 ted, remained within the tube in a gafeous form ', and in that 

 view, I wrapt the tube, which had juft been weighed, in a 

 meet of paper, and placed it, fo furrounded, on the fcale of 

 the balance. As foon as its weight was afcertained, I broke 

 the tube by a fmart blow, and then replaced upon the fcale 

 the paper containing all the fragments. In thofe experiments, 

 in which entire calcination had taken place, the weight was 

 found not to be changed, for all the carbonic acid had already 

 efcaped during the action of heat. But in the good refults, I 

 always found that a lofs of weight was the confequence of break- 

 ing the tube. 



These fads prove, that both caufes of calcination had ope- 

 rated in the porcelain tubes ; that, in the cafes of fmall lofs, 

 part of the carbonic acid had efcaped through the vefTel, and 

 that part had been retained within it. I had in view methods 



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