266 BIOLOGICAL EFFECTS OF RADIATION 



rule, there is no time lag between the excitation and the emission of the 

 fluorescent light. The wave-length of the emitted light is usually longer 

 than that of the exciting light because there is a natural degradation 

 of energy from quanta of large energy to the quanta of smaller energy. 

 The generalization that the fluorescent light is of longer wave-length 

 than the exciting light is known as Stokes' law. There are a few excep- 

 tions to this relation which can be explained as special cases in which 

 additional energy from some other source is involved. Fluorescence 

 caused by the energy-rich ultra-violet light is quite common and it is 

 well known that many substances give ofT visible light when excited by 

 ultra-violet light. A few examples may be cited — fluorescein, eosin, 

 many minerals (particularly if mixed crystals are involved), teeth and 

 various parts of animal tissue. Solutions of quinine when excited by 

 blue light emit red light. 



In all these cases it is assumed that the exciting light displaces an 

 electron in the absorbing molecule and that this electron then returns to a 

 state of lesser energy. If the electron returned to exactly the same 

 state from which it started by a single step, the wave-length of 

 the emitted light would of course be exactly the same as that of the 

 exciting light and the fact of the emission would not ordinarily be 

 detected. 



CHEMICAL KINETICS 



The extent to which a chemical reaction proceeds can be predicted 

 on the basis of thermodynamics when sufficient data are available. The 

 calculation of equilibrium constants is comparatively simple in principle. 

 It is a much more difficult problem, however, to determine the rate at 

 which this equilibrium is established. Chemical kinetics endeavors to 

 answer the question as to how fast a reaction will go and also strives to 

 learn something regarding the nature of the reaction and the mechanism 

 by which it occurs. In very rapid reactions, such as occur in electrolytic 

 systems (ions) or at very high temperatures, the simple rules of thermo- 

 dynamics and equilibrium constants can be readily applied and they give 

 a completely satisfactory prediction of the results. In slow reactions, 

 including the majority of reactions in organic chemistry and biological 

 chemistry, the speed of the reaction becomes a matter of great impor- 

 tance. Among the reactions of organic chemistry a great many different 

 products may result, all of which are possible according to thermodynamic 

 calculation. The product which is formed in actual practice is the one 

 which is produced by the fastest reaction. It is a matter of experience 

 and technique to accelerate or retard these simultaneously occurring 

 reactions in such a way as to obtain the desired product. For example, 

 catalysts are used which will accelerate one reaction to a much greater 

 extent than other competing reactions. 



