Refearcles on Alumine. 153 



If ammonia be employed to precipitate gelatinous alumine, 

 it is not neceflary to dilute the folution with ft) large a quan- 

 tity of water as when the carbonate is ufed, beeaufe the dif- 

 cngagement of the carbonic acid contributes to give to the 

 precipitate a fpongy appearance. But liquid ammonia highly 

 concentrated can itfelf furnifh a fpongy alumine, when the 

 folution of alumine is much concentrated, beeaufe the ele- 

 ments of the precipitate have not fufficient room to float 

 freely in the liquor, and to difpofe themfelves in fuch a man- 

 ner that the light may not be refra&cd in paffing from one 

 molecula to another. 



Spongy alumine and gelatinous alumine, dried at the tem- 

 perature of the atmofphere, contain the fame quantities of 

 water, as I mall foon prove. But the former, on account of 

 its minute divifion, abandons, at the firft degrees of incan- 

 defcence, all the water it can contain ; while the latter, more 

 compact, retains a part at the higheft degree of heat that our 

 furnaces can produce. 



A hundred parts of fpongy alumine lofe in weight, at a 

 red heat, lower than that which makes filver enter into fufion, 

 fifty-eight parts; and it does not lofe a greater quantity at 

 130 degrees of Wedgewood's pyrometer, a heat capable of 

 filling iron. 



A "hundred parts of gelatinous alumine, at the firft degrees 

 of incandefcence, lofe forty-three parts of their weight; and 

 only forty-eight parts and a quarter at J 30 degrees of Wedge- 

 wood's pyrometer. 



To demonftrate that fpongy alumine and gelatinous alu- 

 mine, when dried at a temperature between 10 and 20 de- 

 grees of Reaumur, contain the fame quantities of water, I 

 difiblved in nitric acid 100 parts of gelatinous alumine, dried 

 at a temperature between fi and 20 degrees of VVedgewood's 

 pyrometer, affilting the folution by mixing it with potafli at 

 a filling heat. The purified alumine was precipitated with 

 the precautions already prefcribed for obtaining this earth 

 under the fpongy form. It was then dried at a temperature 

 between the 1 5 th and 20th of Wedgewood's pyrometer, and 

 it weighed then only twenty-five parts. A hundred parts of 

 gelatinous alumine, dried at the firil degrees of incandefcence, 

 retain, then, about fifteen parts of water; which agrees with 

 the remits above announced. 



I have entered into theft details, which are doubtlefs mi- 

 nute, onlv beeaufe great errors may have been committed in 

 chemical analylcs by eftimating, as hitherto appear? to have 

 been the cafe, the quantity of alumine by the weight of this 

 dried earth obtained at that red heat which gem-rally indicates 

 the firft degrees of incandefcence. This method is exacl only 

 4 1 when 



