298 Dr. Priestley’s Experiments 
contain only 0.0084 gr. of plumbago; and if we fuppofe with 
Mr. Kirwan, that an hundred cubic inches of fixed air con- 
tains 8.14 gr. of phlogifton, the fixed air produced in one of 
the above-mentioned proceffes (viz* four- fifths of an ounce- 
meafure) would contain .032 gr. of phlogifton, which is above 
three times more than the plumbago in the iron could furnifh. 
It is evident, therefore, that the quantity of fixed air that I 
found mull: have been formed by phlogifton from the iron 
uniting with the dephlogifticated air in the veffel. 
If, as I have inferred, from burning charcoal of copper in 
dephlogifticated air (fee Experiment, Vol. VI* p. 272.) fixed 
air confifts of 3.45 parts of dephlogifticated air and 1.5 of 
phlogifton, it will be found, that four-fifths of an ounce mea- 
fure of fixed air will contain 0.21 gr. of phlogifton, which is 
much more than on the fuppofition of Mr. Kirwan. 
Another argument againft the antiphlogiftic do&rine may be 
drawn from an experiment which I made upon Pruffian blue; 
if the fmall quantity of fixed air, that may be expelled from it 
by heat, be compared with the much greater quantity which 
is produced when heated in dephlogifticated air. 
Pruffian blue is generally faid to be a calx of iron fuper- 
faturated with phlogifton, though of late it has been faid by 
fome that it has acquired fomething that is of the nature of 
an acid * From my experiments upon it* with a burning lens 
in dephlogifticated air, I fhould infer, that the former hypo- 
thefis is true, except that the fubftance contains fome fixed air, 
which is no doubt an acid ; for much of the dephlogifticated air 
difappears, juft as in the preceding fimilar procefs with iron. 
I threw the focus of the burning lens upon 2 dw. 5 gr. of 
Pruffian blue in a veflel of dephlogifticated air, of the ftan- 
dard of 0.53, till all the colour was difcharged. Being then 
weighed, 
