$o Refcarcbes on Alummc. 



that the quantity of the carbonic acid gas difengaged was 

 not conllant in the different operations, and that it appeared 

 to me to vary in the ratio of the concentration of the alkaline 

 carbonats, and in particular of the fuperabundanr quantity 

 which I added of thefe carhonats to decompole the fulphats. 

 The precipitates in all thefe cafes were waflied in fuch a 

 manner as to avoid every error in this ratio. 



V. A hundred parts of the fame fulphat of alumine, pre- 

 cipitated by carbonat of ammonia in excefs, furnifhed only 

 23 parts of alumine dried at the temperature of the atmo- 

 fpnere. The earth dilTolved in acids with a difengagement 

 of carbonic acid gas, but it loft in aerian acid only one and 

 a half per cent, during the operation. 



Bertholletj in his ingenious refearches on affinities, found 

 that alumine precipitated by ammonia from potaftied fulphat 

 of alumine full retained potafh and acid. To purify this 

 earth, I dincjved it in muriatic acid, and precipitated it again 

 by carbonat of ammonia. The 23 parts of alumine were, by 

 this purification, reduced to 21 parts. They loft then one 

 and a half per cent, of carbonic acid gas by their folution in 

 the acids. They conld not form oclaedral cryftals with ful- 

 phuric acid ; and the imponderable quantity of muriatic acid 

 which they retained could not produce any fenfible error. In 

 the following experiments I (hall give the. name of pure alu- 

 mine to the earth obtained from alum iubjecHed to this double 

 precipitation. 



I divided the pure alumine into two parts, one of which, 

 was pulverifed, and the other left in its natural ftate of aggre- 

 gation. They were cxpofed to the open air for eighteen months 

 on a tablet in my laboratory, after which period the pulver- 

 ifed alumine produced no eftervefcence*. The alumine which 

 had not been pulverifed, and which was placed in the fame 

 circumftances, produced a very fenfible one. This phaeno- 

 nienon inclined me to think that the eftervefcence of the alu- 

 mine, which I believed to be in the ftate of carbonat, arofc 

 from the carbonat of ammonia retained in the unpulverifed 

 alumine, and which had been volatilifed in the other, all the 

 parts of which prevented free accefs to the external air. I, 

 indeed, found that potafh triturated with the eflfervefcent alu- 

 mine, difengaged it from ammonia, and did not produce the 

 fame effeel on the fame earth when pulverifed. 



* This faft is contrary to that announced by Fourcroy. {Syjltme des 

 Connoljpinres Chimiqu-cs, torn. ii. p. 145.) He lays alumine gradually takes, 

 up the carbonic acid ^as () f the atmofpherc. It would be of importance to 

 know the details of th s obfervation, which, in my opinion, was never made 

 but by the author heie quoted. 



VI. I 



