some time before the whole of the oxygen gas is con- 
sumed ; and, consequently, enough of that gas might 
remain to support vegetation, especially that of the 
hardier plants. But a still greater source of fallacy 
is, probably, to be found in the carbonic acid formed 
in these processes, which, if retained in contact with 
the plant, might, as we shall hereafter see, be de- 
composed by Light, and thus furnish oxygen gas to 
carry on the growth of the plant. It is worthy of 
remark, that Mr Scheele never found foul air to be 
rendered salubrious by vegetables, even when placed 
in the light of the sun ; but, in all his experiments, 
he carefully removed the, carbonic acid by washing 
it in milk of lime before he placed the vegetables in 
it *. And, in accordance with this fact, Dr Priestley 
himself observes, that when he employed air that had 
been vitiated by iron-filings and brimstone, or by ni- 
trous gas, it did not fail to kill the plant f 
248. Dr Priestley's next attempts to establish the 
purifying powers of plants were made on air that had 
been injured by animal respiration. Into a jar, near- 
ly filled with air rendered noxious by mice dying in 
it, he put a sprig of mint, on the 20th June ; and 
another portion of the same air he put into a phial, 
which was placed by the side of the former. In se- 
ven days, a mouse lived five minutes without uneasi- 
ness in two and a half ounces of the air in which the 
plant had grown ; but died in two or three seconds, 
in the same quantity of the air of the other phial. 
* On Fire and Air, p. 37* 163. 
f Obs. on Air, vol. iv. p. 301. 
c 2 
