decompounding fixed Air. 305 
This experiment seems decisive, that the charcoal afforded 
in the former ones was derived from carbonic acid. 
My next experiment was with caustic alkali and phosphorus. 
The caustic vegetable alkali I employed was blackish, partly 
from a very small quantity of calx of iron, and partly, I 
think, from other causes which I do not understand ; and I 
was not able to prepare myself, or obtain from others, fixed 
caustic alkali in a solid form which was colourless. It also 
always contained a small quantity of carbonic acid. I in- 
troduced into a glass tube 50 grains of phosphorus, and 150 
of pulverized caustic vegetable alkali, previously found to con- 
tain three ounce measures of carbonic acid in each 100 grains. 
This charge was exposed to heat, as in the former experi- 
ments. On breaking the tube, when cold, the alkaline matter 
was blacker than before : a little of it thrown into hot water 
emitted bubbles of phosphoric air, but not in cold water : in 
rubbing off this alkali from the sides of the tube some pieces 
of it took fire. I dissolved as much as I could of this black 
alkaline matter, by pouring boiling water upon it on a filter : 
a greenish lixivium passed through first, then a less coloured 
alkaline liquor ; and last of all, limpid water. A residue left 
upon the filter being dried, weighed ten grains ; it was a 
blackish brown, impalpable powder, at least five times speci- 
fically heavier than the charcoal obtained in the preceding ex- 
periments. 
(a) Six grains of this residue upon a thin plate of iron, 
heated over a candle, burnt with a green and blue flame, 
emitting a somewhat arsenical odour, and it did not remain 
ignited after the flame ceased. A coal-like matter was left, 
which weighed three grains. / 
