AND OF PLANTS UPON THE ATMOSPHERE. 
163 
inents I have hitherto made, that both the exhalation and the absorption of moisture 
by plants, so far as they depend upon the influence of light, are affected in the greatest 
degree by the most luminous rays, and that all the functions of the vegetable economy, 
which are owing to the presence of this agent, follow in that respect the same law. 
And this is just what we ought to expect, if we suppose that light acts upon the vege- 
table, as it does upon the animal kingdom, in the character of a specific stimulus ; for 
we all are sensible, that the vivifying influence of light upon ourselves is in proportion 
to its brightness ; nor is it uninteresting to remark, that rays of the very description 
which most abound in solar light, are at once the most cheering to the animal crea- 
tion, and the most conducive to the growth and well-being of the vegetable. 
We have already seen, that even that most intense form of artificial light which is 
emitted from incandescent lime, produces no sensible influence upon plants ; and we 
are reminded, that the same holds good with respect to animals, by the fact said to 
have been observed by the exhibitors of the oxyhydrogen microscope, namely, that 
animalcules of kinds which used to be speedily destroyed by the too stimulating 
action of solar light, appear to suffer much less from that now substituted, provided 
the water in which they are immersed does not become heated thereby. 
Part II. On the Action of Plants upon the Atmosphere. 
Having now considered the mode in which light may be supposed to operate upon 
plants, I shall next proceed to examine the extent of the changes wrought in the con- 
stitution of the atmosphere by the latter, under the influence of this agent. 
I say the extent of the changes produced, because no one denies the nature of this 
operation, or questions the fact, that carbonic acid is really decomposed, and oxygen 
given out, by the green parts of plants under certain circumstances. But between 
the original views of Priestley, who saw in this process a counterpoise to the effects 
of animal respiration and the like, and those of Ellis, who did not admit it even as 
an equivalent to the opposite tendency of vegetable respiration, as carried on during 
the absence of solar light, a wide difference exists, and hence some fresh investiga- 
tions seem necessary with reference to this question. 
On perusing the account, which Mr. Ellis, in the Second Part of his Researches 
on Air, has given of the experiments by which he attempts to establish this latter 
opinion, several circumstances occurred to me, of a nature calculated to throw doubts 
upon their conclusiveness. I may mention, for example, in the first place, the small- 
ness of the volume of air in which his plants were confined ; in the second, the length 
of time that was suffered to elapse before the air underwent examination, owing to 
which it is even stated occasionally that the leaves had begun to fade and drop off ; 
in the third place, the removal in some cases, and the neglect of a due supply in all, 
of that carbonic acid, the decomposition of which would have constituted the very 
source of the oxygen which it was expected to discover. 
Accordingly I kept constantly in view these three essential conditions, and 
y 2 
