552 PRINCIPLES OF GENERAL PHYSIOLOGY 



In the first place, we find that the rate of the reaction does not follow the 

 simple law of mass action. This is due to the fact that it is controlled by the 

 amount of light energy absorbed per unit time and not by the actual number of 

 molecules present. An instructive case is that of the oxidation of quinine by 

 chromic acid in light, as investigated by Luther and Forbes (1909). The order of 

 this reaction depends on the colour of the light; violet light is only slightly 

 absorbed and the reaction is unimolecular, ultra-violet is strongly absorbed and 

 the order is very much lower ; since this light is totally absorbed, the rate of the 

 reaction is independent of the concentration of the reacting substances. 



A curious result of this fact is that the order of the reaction depends on the thickness of 

 the layer of solution through which the light passes. In a thick layer the relative amount of 

 violet light absorbed increases, so that the order of the reaction is higher than in a thin 

 layer, where the violet light is scarcely absorbed at all. We have then a reaction, whose 

 order depends on the shape of the vessel in which it takes place. 



In practice, it is found that the majority of reactions brought about by light 

 are of the nature of oxidations or reductions, that is, reactions in which changes of 

 valency take place, as we shall see in more detail in the next chapter. But all 

 kinds of reactions are also to be met with. 



As the phenomena of resonance imply an increase of free energy in the system 

 concerned, it is not unexpected to find that when chemical change takes place it is 

 in the direction such that the resonance is diminished or ceases, according to the 

 second law of energetics. 



The mechanism of this resonance process is, according to Luther (1908), 

 essentially as follows, on the basis of the electro-magnetic theory of light, the 

 electronic (atomic) nature of electricity, and the electrical nature of chemical 

 combination. Compounds consist of molecules or atoms smaller than themselves 

 and between these constituent elements, that is, between their electrons, there 

 is an electric field. The stronger the field the firmer the combination, or the 

 more inactive the compound, and the shorter the period of vibration of the 

 (negative) electron ; that is, the further in the ultra-violet the absorption bands 

 lie, the more stable the compound. Conversely, the further the absorption band 

 lies towards the red end, the more sensitive is the compound to light. Thus 

 anthracene, with its bands in the ultra-violet, is less sensitive than chlorophyll, 

 with its band in the red. Researches by Luther and Nikolopoulos (1913) on a 

 series of organic compounds confirm this view. 



Further, when the periodic alternating electric field of light acts on the 

 electrons, resonance comes into play, their energy content rises, and would do so 

 indefinitely if it were not changed into heat by some kind of " damping " (possibly 

 due to impacts). In the work of Luther and Nikolopoulos, referred to above, 

 it was found that the steeper and higher the absorption curve, the more 

 sensitive to light. Such a curve, in fact, means a small degree of damping, and 

 great amplitude of vibration of the electrons set in motion by light. 



The extent of the loss by damping determines the efficiency of the photo-chemical 

 process, which may be very high, as we shall see when discussing the chlorophyll 

 system. The following illustration given by Luther (1908) may perhaps make 

 clear the possibility of a high efficiency. The substance sensitive to light is 

 compared to a reservoir into which air is pumped, the compressed air representing 

 the radiant energy of light. The pressure of the air (i.e., the energy of resonance) 

 inside the reservoir would rise indefinitely except that a hole, D, is provided, 

 through which air can escape, this loss representing the change to heat by damping. 

 Suppose, however, that there is another hole, C, of adjustable aperture, through 

 which air can escape. This represents the change of part of the energy of 

 resonance to chemical work. The pressure will then decrease according to the 

 area of C, and with it, the amount of escape through D ( = degradation to heat). 

 If C is made very wide, all the air pumped in escapes through it, and none 

 through D. 



Resonance energy thus tends to decrease, either by change of rate of 

 vibration of the resonator, or by increase of damping. Hence, in light of a given 

 frequency of vibration, systems insensitive to it arise from those sensitive to it, 



