THE ACTION OF LIGHT 553 



Although light energy cannot act unless absorbed, it does not follow that, 

 when absorbed, chemical change always results ; for example, acetic anhydride has 

 an absorption band between wave lengths 320 and 240 pp, which is associated 

 with decomposition. But it also absorbs the extreme ultra-violet, apparently by 

 means of its CH 3 group, and resonance of this group does not lead to change. 



A fact common to all photo-chemical reactions may be mentioned here, namely, that the 

 action of light is similar to that of a high temperature. The dissociation of carbon dioxide and 

 of hydrochloric acid, the conversion of oxygen to ozone, and the polymerisation of anthracene 

 may be referred to. For certain theoretical conclusions, drawn by Warburg from this fact, 

 Weigert's monograph (1911, p. 94) may be consulted. 



A theory" has been developed by Bodenstein (1913) according to which the first effect of 

 light is to decompose a group into electron and electro-positive remainder. Each of these gives 

 rise subsequently to chemical changes of a particular kind. 



PHOTO-CHEMICAL REACTIONS THEMSELVES 



When we proceed to examine the various reactions which occur under the 

 influence of light, we meet -with great variety and complexity. It is well, there- 

 fore, to clear the way somewhat by reference to a not infrequent misconception of 

 the nature of the action of light, in which it is spoken of as being catalytic. The 

 initial phase of all photo-chemical reactions is accompanied by the actual consump- 

 tion of light energy to set in motion a reaction, although it may afterwards proceed 

 with the evolution of energy. We have seen in Chapter X. that a catalyst adds no 

 energy to a reacting system, but merely accelerates the rate at which such a 

 reaction arrives at equilibrium. Further, in many reactions, such as the decom- 

 position of carbon dioxide by the green leaf, the reaction is actually caused to 

 proceed in the direction opposite to that in which it goes naturally at the tempera- 

 ture of the reaction. But, in many cases, a catalyst is formed by the action of 

 light, and this catalyst then proceeds, independently of the light reaction proper, 

 to perform its usual function of accelerating the natural course of the reaction. 

 In this case, contrary to that of the chlorophyll system, the net result of the 

 change is a diminution in the free energy of the system. 



It will, perhaps, assist the understanding of the question if we use an illustration due to 

 Ostwald (1902, II. 1, p. 1087). The necessity of the supply of energy by light is obvious enough 

 in that class of reactions in which the result is an increase in the energy content, but is not so 

 clear when the final result is a decrease. Quantitative measurements show, however, that, 

 even in the latter case, there is more liberation of energy than if the reaction had merely pro- 

 ceeded without light, the extra energy being that obtained from light in the initial process. 

 Ostwald compares the system to that of a wedge-shaped block standing with its narrow edge 

 upwards. In this position, that of " metastable equilibrium," the system would remain 

 indefinitely if undisturbed, although by falling on its side energy would be given out. To 

 cause this to take place, there is necessity for a certain expenditure of energy upon the block 

 in order to tip it over ; in this process, its centre of gravity is raised and the energy required 

 to do this is given out again when the block falls over. Another illustration that might be 

 given is that of a billiard ball lying in the concavity of a clock glass on the top of a tripod. 

 Although energy would be given out by the fall of the ball, supposing that the glass were to 

 melt away, no change takes place naturally unless the ball is first raised over the edge of the 

 glass by the application of a small amount of energy. This energy is given out again, together 

 with that due to its original height, when the ball falls to the table. We may speak of 

 reactions of such a kind as being prevented from taking place spontaneously by the existence 

 of chemical "resistance," which is removed by the energy imparted by light. 



Our further considerations will be facilitated by taking the classification of 

 Weigert (1911, p. 75), together with an illustration of each class. This 

 classification is based on the net result of the reaction, which results from the 

 action of light, not merely on the actual part played by the light energy used. 



We have seen already that these reactions can be divided into two main 

 groups, those resulting in increase of free energy and those resulting in a decrease. 

 Each of these can be further divided. The first group may be either simple or 

 complex, in both cases being completely reversible and similar to an electrolytic 

 decomposition in which the electrodes become polarised, so that the reaction 

 ceases at a certain point. 



1. Simple Reactions with Increase of Energy. In these cases, the reversion on 

 removal of light takes place by the same route as the photo-chemical change. 



