270 BIOLOGICAL EFFECTS OF RADIATION 



rate of the reaction. There is still, however, a complication in the fact 

 that the reactants in the front of the cell are exposed to a much greater 

 intensity of light than those deeper in the cell. In some cases this differ- 

 ence has no effect, but in others it leads to complications. When the light 

 is partially absorbed, it becomes necessary to measure the intensity 

 of the transmitted light as well as that of the incident light, and frequent 

 measurements are necessary if the absorption changes during the photo- 

 chemical reaction. Usually it is necessary to stir a liquid system to 

 maintain uniform conditions and to distribute throughout the whole cell 

 the chemical products produced by the beam of light. In the case of 

 gases the diffusion is so rapid that stirring is unnecessary. 



The order of a photochemical reaction may be determined in the same 

 manner as that described in the preceding section. The rate of the reac- 

 tion at constant intensity of light is determined experimentally. If this 

 rate is the same at all concentrations, the reaction is of zero order. If it is 

 directly proportional to the concentration, it is first order, and if it is 

 proportional to the square of the concentration, it is second order. In 

 the case of chain reactions the situation is complicated and in many cases 

 the determination of the order of the photochemical reaction is without 

 significance. 



Photochemical measurements enable one to determine conveniently 

 the length of chains. The number of quanta absorbed by the reacting 

 system is measured and the number of molecules reacting is determined 

 experimentally. The ratio of the molecules reacting to the quanta 

 absorbed gives a measure of the quantimi yield. In case many molecules 

 react per quantum absorbed the reaction is a chain reaction. 



Photocatalyst. — In many cases a reacting system is unaffected by 

 radiation until an additional material is introduced. Such a material 

 is sometimes called a photocatalyst. It is unaffected by the reaction 

 and may be recovered unchanged. Examples are common. The 

 radiation may pass through the reacting system without absorption but 

 when stopped by the photocatalyst, photochemical reaction may result. 

 The photocatalyst may give rise to an entirely different reaction from that 

 which occurs in its absence, and the way in which it functions may vary 

 considerably. In some cases it forms a loose compound with the reac- 

 tants and this compound is excited by the absorbed light. In other cases 

 the photocatalyst is excited by the absorbed radiation and transmits 

 its energy to a reacting molecule by ordinary collision. 



Induction Period. — Sometimes the catalyst of the photochemical 

 reaction is produced by the reaction itself. Under these conditions a 

 time lag or induction period is observed and the photochemical reaction 

 does not proceed until a considerable period has elapsed. The situation 

 is similar to that of autocatalysis in ordinary chemical kinetics where 

 the catalytic material is produced by the chemical reaction itself. Under 



