156 Rcfe arches on Alumine. 



each experiment about 200 grains of alumine, and a balance 

 fenfible to the twentieth of a grain, as great exaclncfs will 

 be attained as that given by Wcdgewood's pyrometer in its 

 of perfection ; but it is feen, alio, that thefeobfervations 

 cannot be extended beyond the 130th degree of that pyro- 

 meter, becaufe at a higher the lofTes in weight of the alumine 

 are little or not at all fenfible in fmall quantities. 



The method I propofe would be particularly ufeful when, 

 it were withed to determine the degrees comprehended be- 

 tween the 257th degree of Fahrenheit and the wcakeft incan-. 

 defcence; degrees not comprehended in Wedgewood's fcale, 

 and which cannot be, on account of the dilatation experienced 

 by argil before it attains to a red heat* I ihall here mention 

 fome precautions which ought to be employed in order to 

 make thefc obfervations with exactnefs. 



iti, The defiecation of gelatinous alumine at the tempe- 

 rature of the atmofpheie is uncertain : it may not be equal 

 in all the parts of the earth ; it is proper then to pulverize it, 

 and to fet out from a fixed degree of defiecation. For that 

 purpofe, a fmall part muft be expofed to a heat of 257° F. by 

 putting it together with that thermometer into a glafs tube, 

 clofed atone of its extremities, and immerfed in a fand-bath 

 betted by an Argancl's lamp, the heat of which is graduated 

 at pleafure. The lofs of weight fuliained by the alumine, 

 dried at the temperature of 257 , is to be deducted in all 

 operations performed with earth dried at the temperature of 

 the atmofphere. The dried earth may be preferved in a bottle 

 with a o-round (topper. 



I have indicated the 257th degree of Fahrenheit as the 

 lowed term, becaufe, at degrees lower than that of boiling 

 water, the lofs of weight in the alumine dried at the tempera- 

 ture of the atmofphere takes place with great tlownefs. 



To deiiccate two drams and a half I employed 

 Nine hours in a heat of 144" F. 

 Two hours - - 257 

 Half an hour - 482 



The termination of this procefs may be readily perceived 

 when the tube which contains the alumine ceafes to be ob- 

 fcured by the vapours that efcape, 



2d, Jtn operations at degrees lower than incandefcencc, it 

 is proper to weigh the earth at the moment when it comes 

 from the fire, becaufe at the end of an hour or two it takes 

 from the atmofphere a quantity of water, which may produce 

 an error of one or two hundredths. 



XXIV. Som 



