AND OF PLANTS UPON THE ATMOSPHERE. 
159 
under the ammonio-sulphate, whose illuminating power was as 2, whilst its chemical 
influence was as 5, they continued of a pale yellow, scarcely indeed of a shade darker 
than in another case where light was completely excluded. 
I have made some experiments with similar results on the colours of flowers, the 
intensity or depth of which appeared also to depend on the brightness of the kind of 
light that had been allowed admission to them. 
The irritability of the Sensitive Plant was likewise found to be dependent on the 
influence of bright rays, and not of those which act chemically. Six healthy sensitive 
plants were introduced in the beginning of August into an oblong box, with parti- 
tions so arranged, that each pot was in contact with a differently coloured light. In 
five weeks’ time, that which had been exposed to the full light of the sun, transmitted 
through transparent glass, was still excitable, as was the one covered with the orange- 
coloured ; but those which had received merely the portions of light transmitted 
through the copper solution, through port wine, and through a solution of green 
muriate of copper, as well as one which had been kept in entire darkness, lost all 
their irritability. Yet in each case the temperature was kept up to the same point by 
means of a hot-bed. 
The exhalation of moisture from the leaves, and the absorption of it by the roots, 
are the last processes dependent on the action of light to which my attention has 
been directed. The results of my experiments on these two points confirm the same 
genera] inference as that to which the foregoing ones point ; but having met with 
some apparent anomalies, I shall forbear at present to report the numerical results of 
the respective trials made with various glasses. It will be sufficient to lay before the 
Society a statement of the plan on which the experiments were conducted, and to 
particularize one or two which tend to show, that the processes above alluded to are 
probably dependent on the combined action of heat and light, coupled with those 
mechanical influences, which operate upon dead, as well as upon living organic 
matter. 
The method whereby I proposed to estimate the degree, in which the exhalation of 
moisture depended on the quality of the light admitted to the plant, was in itself 
sufficiently simple. It consisted, in placing some plant growing in a pot, in a square 
tin vessel, the margin of which received in a groove a cucumber frame of sufficiently 
large dimensions to inclose it. All communication with the external air was cut off, 
by means of a little oil introduced into the groove into which the edges of the frame 
dipped, and the moisture exhaled was absorbed by concentrated sulphuric acid, placed 
in shallow earthen pots, along with the plant, in the interior of the tin vessel. 
By weighing these vessels, just before the plant was introduced, and immediately 
after it had been taken out, I hoped to ascertain the amount of water that had been 
evolved, and after deducting from the sum total the quantity which had been previously 
found to be given off by the plant in the dark, I concluded that the remainder ought 
to represent the quantity to be set down to the action of light. But as it was impos- 
