on the Vapour of Acids . 297 
had been about fix grains. I repeated the experiment with 
the fame refult. 
When the dephlogifticated air is more impure, the quantity 
of fixed air will always be lefs in proportion. Thus, having 
melted iron in feven ounce meaiures of dephlogifticated air of 
the ftandard of 0,65, it was reduced to 1.6 oz. m. ; and of this 
only one-third of an ounce meafure was fixed air. This, how- 
ever, is much more than can come from the plumbago in the 
iron ; but as the production of this fixed air is by many 
afcribed to this plumbago, it may be worth while to fhevv by 
computation that it is impofiible that it fhould have this origin. 
Both the quantity of plumbago in iron, and the quantity of 
fixed air in plumbago, are much too fmall for the purpofe. 
From half an ounce of the pureft plumbago, I firft got, in a 
coated glafs retort, 13 ounce-meafures of air, of which only 
three ounce meafures were fixed air, the reft being inflamma- 
ble ; then putting it into an earthen tube, 1 kept it feme 
hours in as great a heat as I could produce, and got 22 oz. 
m. more ; and of this alfo only three were fixed, and the reft 
inflammable, and the laft portion was wholly fo. 
But inftead of fuppofing the fixed air that I got to be that 
which was expelled from the plumbago in the iron, I will 
fuppofe that even the whole of this plumbago afforded only 
one of the elements of the fixed air, viz. phlogifton, or that 
which the French chemifts call carbone ; and that this princi- 
ple, by its union with the dephlogifticated air in the veflel, 
forms the fixed air, yet on this moft unfavourable and impro- 
bable fuppofition the quantity will be found to be infufficient. 
If 100 gr. of iron contain, according to M. Bergman, 
0.12 gr. of plumbago, 7 gr. (which is the moft that in any 
of the preceding procefles I converted into, finery cinder) would 
4 • contain 
