170 INACTIVATION OF COMPLEMENT BY LIGHT 



In any case all parts of the radiated material must be exposed to 

 essentially the same light intensity; otherwise the true order of the 

 reaction cannot appear. In the case of serum, which is quite opaque 

 to ultra-violet light, the part of the solution furthest from the light 

 is shaded by the intervening serum. The opacity or absorption 

 index of serum is so great that the back surface of a layer of comple- 

 ment diluted as much as practicable and only 0.2 mm. deep would 

 still receive only 0.9 as much light (of the wave-lengths affecting 

 complement) as the front surface. When the radiated material is 

 a solution it can be vigorously stirred so that each portion is exposed 

 to high and low light intensity alternately. In the case of a serum 

 solution stirring is the simplest method of exposing all the solution 

 to the same average light intensity. 



Even with the two precautions indicated above, it is possible to 

 determine the kinetics of a photochemical process only under certain 

 restricted conditions which are not under the control of the experi- 

 menter, but inherent in the solution investigated. The existence of 

 these conditions must be established by appropriate experiments 

 before conclusions can be drawn as to the nature of the photochemical 

 process itself. There are two points to be established: the absence 

 of conditions causing progressive changes in the amount of effective 

 light; and the neghgibihty of diffusion as a limiting factor in con- 

 trolling the rate of reaction. 



II. 



If a reaction appears to be monomolecular, as complement photo- 

 inactivation does, it is always possible that this is the case simply 

 because the reaction occurs with great velocity at some particular 

 region, such as for example the walls of the containing vessel. The 

 reaction will then appear to be monomolecular solely because the 

 velocity of the reaction is limited by the amount of the substance 

 arriving in the reaction field by diffusion, and, as the reaction pro- 

 ceeds, the rate of this diffusion decreases exponentially in accordance 

 with Pick's Law.^ 



^ Fick, A., Ann. Phys. Chem., 1855, xciv, 59. 



