69 
experiments. On causing a plant to vegetate in pure water, sup- 
plied with common air and exposed to light, the carbon of the 
plant increased in quantity ; but when supplied with common 
air in a dark situation, it even lost a portion of the carbon 
which it had previously possessed. 
Ill Light, its effects on Plants. Light is necessary to the col- 
our of plants. The experiments of Sennebier and Mr. Gough have 
shown that the green colour of the leaves is not developed, except 
when they are in a situation to absorb oxygen and give out carbonic 
acid. Though the experiments of different philosophers agree as 
to the influence of vegetation on the air in sunshine and during the 
night, very different opinions have been expressed both as to the 
phenomena occasioned by diffused daylight, and concerning the 
total effect produced by plants on the constitution of the atmo- 
sphere. Priestly found that air vitiated by combustion, or the 
respiration of animals, and left in contact for several days and 
nights with a sprig of mint, was gradually restored to its origi- 
nal purity ; and hence he inferred that the oxygen gas, con- 
sumed during these and various other processes, is restored to 
the mass of the atmosphere by the agency of growing vegetables. 
This doctrine was confirmed by the researches of Ingenhousz and 
Saussure, who found that the quantity of oxygen evolved from 
plants by day exceeds that of carbonic acid emitted during the 
night; and Davy arrived at the same conclusions as Priestley. 
But an opposite opinion has been supported by Mr. Ellis, who 
from an extensive series of experiments, contrived with much 
sagacity, inferred that growing plants give out oxygen only in 
direct sunshine, while at all other times they absorb it ; that 
when exposed to the ordinary vicissitudes of sunshine and shade, 
light and darkness, they form more carbonic acid in the period 
of a day and night, than they destroy ; and, consequently, that 
the general effect of vegetation on the atmosphere is the same 
as that produced by animals. The recent experiments of Dr. 
Daubeny appear decisive of this question. He has convinced 
himself that in fine weather a plant consisting chiefly of leaves 
and stems, if confined in the same portion of air night and day, 
and duly supplied with carbonic acid gas during the sunshine, 
will go on adding to the proportion of oxygen present, so long 
as it continues healthy, at least up to a certain point, the slight 
diminution of oxygen and increase of carbonic acid which take 
135 lUCTABICM. 
