554 PRINCIPLES OF GENERAL PHYSIOLOGY 



The simplest one is the polymerisation of anthracene to di-anthracene by ultra- 

 violet light, as investigated by Luther and Weigert (1905). The stable 

 condition in the dark is, at ordinary temperatures, that of pure anthracene. If we 

 start with pure di-anthracene in the dark it changes spontaneously to anthracene, 

 at a definite rate. Light causes the formation of di-anthracene. But, since the 

 reverse change is unaffected by light, this change of di-anthracene to anthracene 

 proceeds at its natural rate, and this rate increases by mass action as more di- 

 anthracene is formed by light. Under a given intensity of illumination, therefore, 

 as much anthracene is formed by the " dark " reaction as di-anthracene is formed 

 by light in the same time, so that a " stationary condition," simulating- a chemical 

 equilibrium, is arrived at. Contrary to the latter, which would be a permanent one 

 if left to itself, this stationary condition is only maintained by the continuous inflow 

 of light energy, which becomes transformed to heat. It should be noted that, 

 before the stationary condition is reached, part of the light energy becomes chemical 

 energy. Another example is that of the formation of ozone from oxygen by 

 ultra-violet light. 



A striking fact, which may appropriately be mentioned here, is that the temperature 

 coefficient of a light reaction is usually much lower than that of a chemical reaction proper. 

 This follows from the fact that the rate of the photo-chemical change depends on that of the 

 absorption of light, which varies only very slightly with temperature. The position of the 

 stationary equilibrium in the case of anthracene, in relation to temperature, is controlled almost 

 entirely by the change of chemical equilibrium by temperature. The temperature coefficient 

 of the dark reaction (chemical) is 2'8, that of the light reaction, 1 '1 or less. 



2. Complex Reactions with Increase of Energy. These result from the 

 combination of various purely chemical reactions with photo-chemical effects. Their 

 reversal is by a different route from that taken in their production. The most 

 interesting and important of these is that of chlorophyll and carbon dioxide, which 

 will be treated of in a special section later. They are the most difficult class to 

 analyse, since the various component reactions proceed both simultaneously and 

 successively. 



Those reactions resulting in diminution of free energy are always complex, as 

 we have seen, and may be divided into two main classes : coupled and catalytic. 

 They are non-reversible, in the sense that they do not change back spontaneously 

 in the dark. 



3. Coupled Reactions with Loss of Energy. In these the products of photo- 

 chemical change are immediately used up in another reaction. As a general 

 scheme, we may take that of Weigert (1911, p. 36) : 



Light 

 (1) A > B. (2) B > C. 



Dark Dark 



B is produced from A in the light, and would quickly return to A if it were not at 

 once used up in the second reaction to form C. It is probable that the oxidation 

 and reduction of alcohols by aromatic substances, observed by Ciamician and Silber, 

 belong to this group. The important properties of the chemical sensitisers must be 

 included also. A plate coated with silver bromide alone and exposed to the light 

 belongs to our first class, so that when a certain amount of free bromine has been 

 produced by light, a stationary, balanced condition is reached, owing to the 

 recombination of silver and bromine, as in the dark. Such a plate would be of 

 little use in photography, being comparatively insensitive. If, however, a 

 substance such as gelatine is present, which combines with the bromine as it is 

 produced, a much greater decomposition of the silver bromide takes place. There 

 is no storage of energy, since the final system consists of brominated gelatine and 

 metallic silver (or sub-bromide). The bromination of gelatine is associated with the 

 giving off of energy, and the product has no affinity for silver. 



An electro-chemical analogy to this group is that of an electrolytic process in 

 which the products are used up in a reaction going on in the solution, so that 

 depolarisation occurs and the current continues to flow. 



4. Catalytic Reactions tvith Loss of Energy. The second group of photo- 

 chemical reactions in which there is diminution of free energy is that in which 



