104 LIGHT, VEGETATION AND CHLOROPHYLL 



from which they would not be able to return to their previous 

 position without receiving a supply of energy from an external 

 source. 



Most often, the "excitation" just mentioned is the effect 

 of thermal agitation. This is so with reactions that can be 

 produced in darkness, but in a photochemical reaction the 

 excitation is provided by hght. 



Finally, a catalyst is a substance which reacts on one of 

 the constituents without excitation, or with very slight 

 excitation, and provides an intermediary product which is 

 capable of reacting, in its turn, with the other constituent to 

 give the expected combination, the catalyst being restored to 

 its intial state. 



Excitation, whether it be thermal or photochemical, has 

 the effect of bringing the molecules into a state in which 

 they have more energy, called the "activated" state. This 

 activation may be a change in the electronic configuration of 

 the molecule; the possible configurations form a discontinuous 

 series of quantified states of increasing energy. It may be an 

 ionization, i.e., the separation of an electron torn away from 

 the molecule, or a dissociation, i.e., a sphtting of the molecule 

 into two parts, atoms or groups of atoms, or again, more 

 simply, an activation of the internal vibrations and rotations 

 within the molecular structure. 



The result is always the creation of a state of greater 

 energy, a state in which the molecule can overcome the 

 resistances which oppose the chemical reaction. 



To return to the analogy of the billiard balls, let us imagine 

 that the edges of the table are raised so that the balls cannot 

 fall unless they receive an impulse setting them in motion with 

 sufficient energy to enable them to surmount the obstacle. 



Thermal activation differs from photochemical activation. 

 The first is due to thermal agitation. We know that the 

 molecules of a gas are in continual motion. They have kinetic 

 energy and internal energy, and the higher the temperature of 

 the gas the greater is their average total energy. It is almost 

 zero at —273° C, a temperature that is called for this reason 



