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certain compounds in the plant. I once suspected 
that all the carbonic acid gas produced by plants 
in the night, or in shade, might be owing to the 
decay of some part of the leaf or epidermis ; but 
the recent experiments of Mr. D. Ellis are op- 
posed to this idea ; and I found that a perfectly 
healthy plant of celery, placed in a given portion 
of air for a few hours only, occasioned a produc- 
tion of carbonic acid gas, and an absorption of 
oxygene. 
Some persons have supposed that plants exposed 
in the free atmosphere to the vicissitudes of sun- 
shine and shade, light and darkness, consume more 
oxygene than they produce, and that their perma- 
nent agency upon air is similar to that of animals ; 
and this opinion is espoused by the writer on 
the subject I have just quoted, in his ingenious 
researches on vegetation. But all experiments 
brought forwards in favour of this idea, and parti- 
cularly his experiments, have been made under 
circumstances unfavourable to accuracy of result. 
The plants have been confined and supplied with 
food in an unnatural manner ; and the influence of 
light upon them has been very much diminished by 
the nature of the media through which it passed. 
Plants confined in limited portions of atmospheric 
air soon become diseased ; their leaves decay, and 
by their decomposition they rapidly destroy the 
oxygene of the air. In some of the early experi- 
ments of Dr. Priestley, before he was acquainted 
with the agency of light upon leaves, air that had 
supported combustion and respiration, was found 
purified by the growth of plants when they were 
