182 ESSAY XIII. 



occupied the space of 8 oz. of water ; it contained no aerial 

 acid, but extinguished flame ; neither in the detonation of 

 brimstone is there any aerial acid obtained. But, in order to 

 remove all doubts and objections, I contrived to decompose 

 plumbago without nitre, (c) For this purpose I repeated 

 the experiment already mentioned with the acid of arsenic 

 (Sec. I.). I applied an empty bladder to the retort instead 

 of a receiver. When the arsenic rose into the neck of the 

 retort, the bladder was distended, and I obtained pure aerial 

 acid, (d) I mixed four parts of calcined quicksilver with 

 one part of pulverised plumbago, and distilled this mixture 

 in the same manner as before. The quicksilver was reduced, 

 and the bladder distended with air. This air was aerial acid, 

 and mixed with one- third of pure air. (e) One part of 

 litharge reduced to glass, 1 and then ground to powder, mixed 

 with two parts of plumbago, was reduced in the retort, and 

 the bladder contained pure aerial acid. (/) One part of 

 powdered plumbago, mixed with caustic fixed alkali, and 

 exposed to distillation by a strong fire, yielded inflammable 

 air. The mass remaining in the retort had lost its causticity, 

 and made a strong effervescence with acids. 



SECTION VIII. 



Hence I am convinced that plumbago is a kind of 

 mineral sulphur or charcoal ; the constituent parts of which 

 are aerial acid and a considerable quantity of phlogiston. 

 The small quantity of iron can hardly be taken into the 

 account ; for (1) it seems to be mixed with it only mechanic- 

 ally ; and (2) I have treated plumbago, from which I ob- 

 tained, after calcination, a little more ochre of iron than 



1 Litharge generally containing a little aerial acid, it was requisite to 

 fuse it first, in order to separate the air. 





