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XXIII. Mefearches on Alumine. ReaJ Dec. i3, 1 800, in the 

 Society of Phyjies and JSatural Hijlory at Geneva. By 

 TllHODOllE D£ SAUSSURE. 



[Concluded from p. 3C] 



On the different Degrees of the Defecation of pure Alumine 

 in its different States. 



XjLLUMINE, precipitated by ammonia and by the carbo- 

 nate of ammonia, exhibits itfelf under two very different af- 

 pe6ls, according to the quantity of water in which the alu- 

 minous fait is difToIved, though the weight of the precipitates 

 be the fame when they are dried in a temperature of from 

 65 to 75 of Fahrenheit. 



If the quantity of water does not exceed what is ncceffary 

 for the foiution of the aluminous fait, a white earth is ob- 

 tained, light, friable, exceedingly fnongy, and which ad- 

 heres to the tongue. 1 {hall diltinguilh it by the name of 

 fpongy alumine. 



But if the aluminous fait is difTolved in a very large quan- 

 tity of water, there is obtained, after the precipitate has been 

 dried at the above temperature, a tranfparent mafs% yellow 

 and brittle, which when in fragments is fomewhat volumi- 

 nouS; breaks or fplits in the hand as fulphur does in the like 

 cafe; it has afmooth and conchoid-formed fra&ure, without 

 any earthy afpeel, does not adhere in any manner to the 

 tongue, and neither f wells up nor refolves itfelf into water : 

 it prefents a volume of only a tenth or a twelfth of what the 

 fame weight of the fpongv alumine does, and refembles gum 

 arabic or dried jelly. I ihall give it the name of gelatinous 

 alumine, 



* Opacity is not, then, eiTentinl to alumine, but when, in this or in the 

 gelatinous hate, it has been expofed to a violent heat. I do not know 

 whether it was in this ttare that it was confidered by Fourcroy, when he 

 lays, in his S/Jleme des Connoijfdnces Chimiques, that alumine appears 

 opaque in its ultimate molecular; that it reflects the light entirely ; and 

 that it communicates that property to Oones, into which it enters in a large 

 quantity. But there are many exceptions which deftioy this general rule. 

 I (hall remark, that alumine, confidered either in its fpongy (late or in 

 rh'ir of its complete deficcation, appears opaque only by the divjfion of its 

 parts. With a good microfcope, which magnified 200 times, I obferved 

 gelatinous alumine in fine powder, after being expofed to a heat expreffed 

 by 170 degrees of Wedgcwood's pyrometer. It was tranfparent in' its 

 fmallcft moltcul??, though the powder of alumine had experienced neither 

 agglutination nor fufion. The fapphire, the. topaze, the cymophane, and 

 the ruby, contain more alumine than is found in any opaque Hone or rock 

 V. the furface of the earth ; and yet thefc gems are tranfparent ! By being 

 too eager to eftablifh general rules, v.c fall into confuiion. 



If 



