76 THE CHEMICAL ACTION OF LIGHT. 
most conspicuous absorption-band of the chlorophyll spectrum 
are the most active. This is the more intelligible since it may be 
inferred that the rays are converted into chemical work after 
absorption. ‘The view that it is the rays absorbed by chlorophyll 
which promote the decomposition received important confirma- 
tion by the very ingenious experiments of Engelmann on the 
action of light on bacteria. 
Naturally, there must also be a relation between the intensity 
of the light and the decomposition of carbonic acid. ‘here must 
be a minimum intensity below which this process does not take 
place, and, as Sachs points out, there probably is an optimum 
intensity above which the activity of the process decreases ; but it 
is questionable whether this optimum is ever reached in sunlight, 
at all events in this country. 
As might be expected, rays of light of different degrees of 
refrangibility have a different influence on the increase of weight 
of the organic substance of plants. And as light exercises so 
great an influence upon the building up of green plants, it may 
be inferred that it must indirectly affect the absorption of 
mineral food-materials by the roots; this actually is the case, and 
this process is also dependent upon the refrangibility of the rays 
of light. The following table shows the extent to which the 
different parts of the spectrum affect this absorption. The 
experiments were made by growing peas covered with glasses of 
different colours, but as the light which passed through was in 
no case monochromatic, the results are only relative. 
Plants grown in x Reis in Increase in 
ganic substance. | ash-constituents. 
| ee 
Shiva) Tel Nie ea Aso oedeisdcahaomor 100 100 
Red light .. 2... sues e cee eee eens 35°5 41'4 
VAAN APA tre c8 oe aC Boma an clge ODOR 82°6 62°0 
Blikens peg uct eerariociciaerae steerer he | 22°4 33°3 
Witlete so Sine tehertacal cheba re pate 14°5 5°3 
