Of Plumbago. 159 



It is too foft to ftrike fire with fteel : its 

 fpecific gravity is from 1,987, to 2,267. 



It is infoluble in the mineral acids. 



In a ftrong heat and open fire it is wholly 

 volatile, leaving only a little iron, which 

 feems to be only accidentally found in it, 

 and a few grains of {ilex. 



The ufual fluxes do not effedt its fufion. 



It is decompofed by detonnation with ni- 

 tre, in a red hot crucible. 



According to Mr. Scbeele, who firft dif- 

 covered the nature of this mineral, i part 

 plumbago requires 10 of nitre to decompofe 

 it, whereas i part charcoal requires but 5 of 

 nitre : hence it contains twice as much phlo- 

 gifton as charcoal does. By receiving the air 

 arifingfrom its decompofition in a bladder, he 

 found ~ of that air to be abforbable by lime 

 water, and confequently to be fixed air, and 

 in the remainder a candle would burn : but as 

 nitre itfelf might afford fixed air by com- 

 buftion, to get rid of all objections, he de- 

 compofed plumbago by fubliming and re- 

 ducing the arfenical acid, after mixing it with 

 plumbago ; and thus alfo he obtained fixe4 

 air. It is probable that 100 grains of it con- 

 tain 



