ENERGY, PHOTOCIIEMICALS AND BRIGHTNESS 31 1 



important of these the process of photosynthesis in plants 

 furnishes an excellent example. The maximum for this 

 process lies in the red very near the Fraunhofer line C. 

 This is not solely due to the fact that the rays in this 

 region are more readily absorbed than those in the adjoin- 

 ing regions, for the violet rays are also absorbed, and here 

 there is no appreciable effect on photosynthesis. In 

 solutions which contain ferrocyanide or certain other 

 coloring matter the longer waves are also more effective 

 than the shorter, and pure ozone, which is changed to 

 oxygen only in the ultra-violet, is similarly acted upon by 

 the visible rays if chlorine be added. These various 

 examples inevitably lead to the conclusions that while the 

 shorter rays may induce chemical changes in more sub- 

 stances than the longer, they cannot be considered as the 

 only actinic rays. The relative efficiency of the different 

 rays depends first of all upon one or more of the com- 

 pounds between which the photochemical reaction is tak- 

 ing place, but it also, at least in certain cases, depends upon 

 the presence of substance in which no apparent change is 

 taking place. 



Many of the photochemical reactions are exothermal. 

 For example, the light conditions which induce the ful- 

 gides to become dark are much more effective in lower 

 temperature than in higher. According to Stobbe it 

 requires nearly ten times as much light energy to produce 

 a given change at 100° as it does to produce the same 

 change at 87°. A decrease in heat energy , therefore, pro- 

 duces the same effect as an increase in light energy, a 

 statement which at first thought appears self-contradictory. 

 As a matter of fact, however, it merely demonstrates the 

 independence of these two forms of energy in producing 

 chemical reactions. 



It is evident from what has thus far been presented 

 that the actinic distribution in the spectrum is not 

 proportional to distribution of energy. There are many 

 well-known photochemical reactions which occur only in 



