Oa the Food of Plants. 
515 
by whose researches it appeared rather doubtful whether, after 
all, plants do really possess this purifying power with respect to 
the air. In experiments connected with subjects like the present, 
the greatest possible care and foresight are required to guard 
against accidental sources of error, which beset the observer on 
every side. The discrepant results obtained by so many different 
individuals abundantly testify to the truth of this observation. 
For example, during all experiments of this nature, every pre- 
caution must be taken to keep the plant in a healthy state. If it 
dies, or any portion dies, or even becomes diseased, its proper 
functions diminish in energy, and even cease, and decomposition 
commences — a process whose constant concomitant is the dis- 
appearance of oxygen, and its change into carbonic acid. 
We owe to Professor Daubeny a most valuable set of experi- 
ments in relation to this matter, which appear to demonstrate the 
point, so far as it is capable of demonstration by such means. 
In these researches bell-jars of large dimensions were em- 
ployed, dipping into mercury, covered with a little water, con- 
tained in a kind of double cylinder of iron, the plant or branch 
experimented upon being secured within, and cut off from the 
atmosphere by an ingenious contrivance described in the memoir. 
The jar being so arranged, and the plant introduced, a certain 
portion of carbonic acid was supplied, and the air of the appa- 
ratus carefully examined, and the relation of the carbonic acid, 
oxygen, and nitrogen, carefully determined. During the experi- 
ment this analysis was repeated from time to time, fresh portions 
of carbonic acid being added as that substance became decom- 
posed. The result of a long series of experiments on a great 
variety of plants, and under varied circumstances, are given in 
tables in the memoir referred to. They are all (with the excep- 
tion of one or'two cases, in which thick-leaved, fleshy plants were 
concerned) decidedly in favour of the view advocated. The fol- 
lowing are the details of two experiments : — 
" The jar contained about 600 cubic inches of air. The plant experi- 
mented upon was the common lilac. The proportion of carbonic acid 
in the jar was each morning made equivalent to five or six per cent. 
The first day no great alteration in the air was detected ; but on the 
second day, by eight in the evening, the oxygen had risen to 26 ' 5 per 
cent. In the morning it had .sunk to 26, but by two p.m. it had again 
risen to no less than 29 "75, and by sunset it had reached 30 per cent. 
At night it sunk one-half per cent. ; but the effect during the day was 
not estimated, as the sickly appearance which the plant now began to 
assume induced me to suspend the experiment. 
" In a second trial, however, the branch of a healthy lilac growing in 
the garden was introduced into the same jar, and suffered to remain 
until its leaves became entirely withered. The first day the increase of 
oxygen in the jar ^Yas no more than 0*25 per cent., but on the second it 
