158 
DR. DAUBENY ON THE ACTION OF LIGHT UPON PLANTS, 
Of atmospheric air which had been 
previously absorbed 
together with excess of oxygen . . 
1 68 parts : consisting of / n itLOgen ^6 
J (.oxygen 12 
32 Excess of oxygen . . 32 
Total of gas obtained .... 100 Total of oxygen . . 44 
whereas, when orange-coloured glass No. 5. was employed, we obtained, 
Of atmospheric air > 60'5 Oxygen 12'6 
Excess of oxygen 19 - 4 Excess of oxygen . 19-4 
Total of gas . . 79’9 Total of oxygen . 32‘0 
The two most difficult cases to explain seem to be, first, the evolution of pure nitro- 
gen, and secondly, that of the same gas accompanied with a smaller proportion of 
oxygen than that present in the atmosphere. 
In the instance in which the former was observed, no incipient putrefaction could 
be suspected by way of accounting for its occurrence, for the plants were fresh and 
healthy; and the circumstance that gas is not disengaged at all in the dark, proves 
the evolution of nitrogen to be in both cases a process connected with the same kind 
of action as that to which the emission of oxygen is to be ascribed. 
Perhaps the phenomenon may be better understood by reference to the experiments 
of the younger Saussure, which go to prove; that oxygen becomes fixed in the plant 
in a condition, such as renders it incapable of being withdrawn from the vegetable 
tissue by the air-pump, or by other mechanical: means ; that it there unites with the 
carbon, so as to bring the latter into a fit state for the plant to assimilate it; and that 
it is then again disengaged from its combination, by a process not unaptly compared 
by the late Professor Burnett to the digestion of animals. 
Now for this latter function to be discharged, the stimulus of the more luminous 
rays may be requisite, whilst that of the duller portions of the spectrum may suffice 
for the mere respiration of the plant, or for the elimination of the residuary air. 
Hence when rays of the latter description are alone transmitted, the composition of 
the gas evolved may even indicate a smaller amount of oxygen than that present in 
atmospheric air, because a portion of this element had become combined with certain 
of the carbonaceous principles present in the vegetable tissue, or been fixed in some 
manner within the plant. 
The other processes, enumerated as under the influence of solar light, appear to be 
subjected to the same law, as that by which we have seen the decomposition of car- 
bonic acid in the green parts of plants to be regulated. 
From a few experiments I have made on the secretion of green matter in the 
leaves, I should be led to infer, in contradiction to the results of Senebier, that the 
most luminous rays were most influential ; the orange glass, whose chemical influence 
was as 4, whilst its illuminating power was as 6, quickly imparting to the primordial 
leaves of beans which had just appeared above ground a bright green hue, whereas 
