37 
various kinds, live perfectly well in tainted air, which 
would have instantly killed other animals. He has 
eren been obliged to take plants out of such putrid 
air, on purpose to brush away the swarms of insects 
that infested them, so fast did they multiply in these 
circumstances. Not only, however, did animals 
thus live in air, in which, on Dr Priestley's hypothe- 
sis, they might have been expected to die, but vege- 
tables died in that in which they ought to have 
lived ; for he found, that " when air had been fresh- 
ly and strongly tainted with putrefaction, sprigs of 
mint have presently died on being put into it, their 
leaves turning black ; but if they did not presently 
die, they throve in a most surprising manner *." 
From the facts already stated (244.), it appears that 
the air, vitiated by putrefaction, is restored, in cer- 
tain circumstances, to a state capable of supporting 
vegetable and animal life; and inattention to these 
circumstances suggests at once a probable reason for 
that contrariety of result which these experiments 
afford. 
251. Towards the end of the year 1771 , continues 
Dr Priestley, these experiments did not answer so 
well, the restored air having relapsed to its former 
noxious state. In 1772, he had again the most in- 
disputable proofs of the restoration of putrid air by 
vegetation ; but, in 1778, the experiments were un- 
favourable, for whether they were made with air in- 
jured by respiration, by combustion, or by any other 
means, the air was not rendered better, but worse. 
* Obs. on Air, abridged, vol. iii. p. C 
