32 
to be nearly the proportion of oxygen gas which the 
atmosphere contains. 
242. We have already given (27.) the results 
of Dr Priestley's experiments on this subject, and 
have shewn them to be so much at variance with each 
other, as altogether to preclude the possibility of any 
just inference being drawn from them. It may, 
however, be useful to examine more minutely the cir- 
cumstances in which these experiments were made, 
since we may thereby be led to a discovery of the 
causes of their discordance, and a detection of some 
sources of error, which the state of chemical science 
did not, at that early period, enable this candid phi- 
losopher fully to appreciate. 
243. Dr Priestley had himself at first expect- 
ed, that, " since common air is necessary to ve- 
getable as well as to animal life, both plants and 
animals would have affected it in the same man- 
ner ;" but he found, that, after a sprig of mint 
had grown for some months in a glass jar, standing 
inverted in a vessel of water, the residual air was still 
capable of supporting combustion, or respiration. 
He observes, however, that, in this experiment, the 
root of the mint decayed, and the stalk also, begin- 
ning from the root, and yet the plant continued to 
grow upwards through a black and rotten stem : and 
in this manner a sprig of mint lived, the old plant 
decaying, and new ones shooting up in its place, all 
the summer season *. 
244. But we now know, that, under the decom- 
position of vegetable bodies, various gases are disen- 
Obs. on Air ? abridged, vol. iii. p. 250. 
