AND OF PLANTS UPON THE ATMOSPHERE. 
151 
I felt therefore desirous of putting to the test of experiment the two following 
questions : 1st, whether the several solar rays act upon plants with equal or with dif- 
ferent degrees of energy ; and secondly, whether all the functions of plants that seem 
dependent on the presence of light are affected in the same ratio by similar rays. 
Now the following functions are found to depend in great measure on the influence 
of light. 
1. The decomposition of carbonic acid, and the consequent evolution of oxygen, 
already spoken of. 
2. The green colour of leaves, and other analogous parts. 
3. The expansion or unfolding of the leaves in certain species, the folding of which 
on the close of day constitutes what has been called “ the sleep of plants.” 
4. The irritability belonging to certain other plants, such as the Mimosa pudica. 
5. The exhalation of water from the leaves. 
6. The absorption of the same by the roots. 
The difficulty, however, of comparing the relative intensity of the light transmitted 
by the various coloured media, which were employed in my experiments, induced me 
to content myself with showing, that the effect of light upon plants corresponds with 
its illuminating, rather than with its chemical, or its calorific influence ; and to wave 
the more difficult inquiry, whether its operation upon the vegetable kingdom exactly 
keeps pace with the increase in its own intensity. And in order to show that the 
former is the case, I will in the first place set down the order of sequence of the 
several media, with reference one to the other, in respect to their illuminating, their 
calorific, and their chemical power ; stating at the same time, by means of a diagram, 
the rays intercepted and transmitted by each, as was determined for me by Professor 
Powell ; and afterwards proceed to a statement of their respective influence upon 
plants. 
With regard to the means adopted for estimating these points, I need perhaps only 
remark, that the relative illuminating power of the several media was ascertained by 
the number of thicknesses of wire gauze, which produced a certain definite degree of 
obscurity or indistinctness when interposed ; their relative calorific power, by the 
number of degrees which a thermometer with a blackened bulb was raised in a given 
time by the light transmitted through each ; their relative chemical influence, by the 
time required to reduce paper moistened with a solution of nitrate of silver to a cer- 
tain standard point of discoloration. The figures in the annexed Table therefore 
must be understood to express nothing more than the order of sequence , and not to 
indicate the ratio between the several media in any of the above respects. 
