The Philippine Journal of Science, C. Botany. 
Vol. XII, No. 2, March, 1917. 
THE RELATION BETWEEN LIGHT INTENSITY AND CARBON 
DIOXIDE ASSIMILATION 
By William H. Brown and George W. Heise 
{From the College of Liberal Arts, University of the Philippines, and from 
the Bureau of Science, Manila, P. I.) 
In the absence of complicating factors there is, with many 
photochemical reactions, a direct proportionality between light 
intensity and reaction velocity. This relationship was first pos- 
tulated by Malaguti(i3) and afterward experimentally sub- 
stantiated by Draper. (7) It is expressed in the well-known 
law of Bunsen and Roscoe, (5) as follows: 
E=kit, 
where E is the photochemical effect, i the intensity factor, t the 
time factor, and k a constant. 
Reactions free from complicating factors are by no means 
easy to find. Even with comparatively simple photochemical 
processes, in which all factors can be controlled with some 
degree of certainty, complications frequently arise which cause 
deviations from the Bunsen and Roscoe law. This is well illus- 
trated by the difficulty encountered in securing reactions suit- 
able for actinometric work. With long-continued reactions, high 
light intensities, or great changes in light intensity (22) the 
reaction velocity is often less than strict proportionality would 
require. This discrepancy between cause and effect has been 
determined experimentally by a number of workers, including 
Luther and Weigert,(i2) and has been calculated by Byk.(6) 
It has been determined that a simple relationship does not 
hold in the much-studied case of the exposure of the photo- 
graphic plate. (22) For certain light intensities the photo- 
chemical effect produced .S' is proportional, not to the intensity 
/, but to the logarithm of the intensity, in accordance with the 
equation 
S—k log i. 
For another intensity range the relationship is expressed by an 
even more complicated formula, namely : 
S=Y\og (ft-fC). 
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