as the plant grew, in every respect, in its natural 
Way, except that part of the stem was obliged to lie 
in water, and the shoot was in air confined in a nar- 
row jar *. All the cases in which common air was 
improved by vegetation, he elsewhere says, were those 
in which the roots of the plant were in the ground,, 
and flexible sprigs from them were bent, and made to 
pass, in this manner, through water into the jars- con- 
taining the air f- Most of these experiments, how- 
ever, were made in June and July, and, as they were 
conducted in the open air, it is highly probable that 
they were carried on under a direct exposure to the 
rays of the sun, which we shall her'eafter see to exert 
peculiar effects on the gases produced in this pro- 
cess. 
246. Several sources of fallacy exist in the experi- 
ments which Dr Priestley made to restore the purity 
of air that had been vitiated by the combustion of 
wax, tallow, and alcohol. He placed sprigs of mint, 
and other plants, in portions of this air ; and he found 
that five or six days were sufficient to restore the air, 
when the plant was in its vigour. He farther con- 
sidered this restoration to depend on the vegetating 
state of the plant; for when the fresh leaves only of 
mint were kept in such air, for a long space of time, 
and were frequently changed, no melioration could 
be perceived in it J. 
247. Oh these experiments we may remark, that 
combustion generally terminates in closed vessels 
* Obs. on Air, vol. iv. p. 299. t Ibid. p. 307 
J .Obs, on Air, vol; iii. p. 252. 253. 
