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XIII. On the Action of Light upon Plants, and of Plants upon the Atmosphere. By 
Charles Daubeny, M.D. F.R.S., Professor of Chemistry and of Botany in the 
University of Oxford. 
Received November 3, — Read December 17, 1835. 
The researches of Priestley, Ingenhousz, Senebier, Ellis, and above all of the 
younger Saussure, have long put us in possession of the leading facts appertaining 
to the influence of light upon the green parts of plants ; and Professor Decandolle 
has embodied the substance of all that had been ascertained on this subject, up to the 
year 1831, in his admirable work on Vegetable Physiology. But there appear, by 
the confession of this latter naturalist, to remain certain subordinate questions re- 
specting this same function, which, though perhaps occasionally touched upon by 
the above-cited experimentalists and by others, can scarcely be said to have as yet 
obtained a satisfactory reply. 
The first of these questions relates to the nature of the influence which, in the 
cases alluded to, is assignable to light. As this agent often produces chemical 
changes by its direct action upon inorganic bodies, decomposing saline solutions, 
discolouring oils, and reducing metallic oxides, so it may be supposed to operate 
directly upon the air, and to possess the power of decomposing carbonic acid, when 
this substance is presented to it within the pores of the vegetable tissue. And, on 
the other hand, as light appears to be a specific stimulus to the vital functions of 
animals, so it may be imagined to act in a similar manner on those of plants, thus 
enabling them to secrete from the carbonic acid presented to them the carbon re- 
quired for their nutrition. 
Another point as yet undecided relates to the extent of the influence it exerts 
over the vegetable kingdom ; or, in other words, the degree in which certain pro- 
cesses attributed to its presence are capable of counteracting others that are going 
on at all times, whether light be absent or not. Thus, although it maybe conceded, 
as a fact already well established, that plants purify the air in the sunshine, it still 
remained to be proved by more decided experiments than had hitherto been insti- 
tuted, whether the quantity of oxygen given out by them during the day exceeded 
that absorbed during the night ; and moreover, supposing this latter question an- 
swered in the affirmative, whether the probable excess was likely to be such, as would 
