150 DR. DAUBENY ON THE ACTION OF LIGHT UPON PLANTS, 
afford a counterpoise to the effects produced by animal respiration, combustion, and 
the like*. 
After considering therefore the mode in which light appears to affect the functions 
of plants, I shall naturally proceed to examine the extent of the changes produced by 
the latter upon the air through its influence. 
Part I. — On the Influence of Light upon Plants. 
If of the two modes of considering the operation of light above noticed we adopt 
the second, that is, if light be supposed to affect plants by a specific stimulus, such 
as it exerts on animals, and not in the first instance the air as a chemical agent, it 
ought to follow, that those portions of the spectrum which possess the strongest illu- 
minating power, should exercise upon them the most powerful influence, and produce 
the most decided effects. 
Senebier, however, has stated, that the green colour of leaves, which is sup- 
posed to be connected with the decomposition of carbonic acid and the evolution of 
oxygen, is produced most rapidly under the action of the violet ray-|~; and as the 
latter, from the feeble light and heat it communicates, seems almost inert, with 
reference to the functions of animals, such a circumstance, if substantiated, would 
seem strongly to favour the contrary hypothesis. 
This latter view of the mode in which carbonic acid is decomposed within the ves- 
sels of the plant, would likewise be somewhat confirmed, if it should appear, that whilst 
the above process was most favoured by violet light, other functions, which are af- 
fected by the presence of this agent, but which evidently depend upon a process 
taking place in the vessels of the plant, are influenced in proportion to the luminous- 
ness of the ray ; whilst, if the same law were found to prevail in both these cases, 
and if all the processes alluded to could be proved to go on most rapidly under the 
influence of the darkest and most refrangible portion of the solar spectrum, a curious 
difference between the mode of its operation upon the vegetable and animal kingdoms 
might then be suggested. 
* I am aware it may be urged, that the quantity, of carbonic acid added, and of oxygen subtracted by these 
latter means within any moderate period of time, is in itself so small compared with the entire bulk of the 
atmosphere, that we must not argue, because the constitution of the latter has appeared to continue uniform 
ever since accurate methods have been devised for determining it, therefore that no change can be taking place 
insensibly in the proportions of its ingredients. 
Still, however, when we recollect, how many ages have elapsed since the present races of animals were 
created, and how many more since the existence of others, which, although extinct, appear from the analogies 
of their structure to have carried on the same respiratory process which those now in existence fulfill, and 
therefore could not have endured an atmosphere much more highly charged with oxygen than the present one, 
we cannot help feeling, that nature must have some means at her disposal, by which the purity of the atmosphere 
is restored, and its constitution thus maintained without alteration. 
f Mem. de Phys. Chim., tom. ii. p. 55. 
