LIGHT AND VEGETATION 107 



Thus, the molecules excited by the absorption of a photon, 

 having absorbed its energy, very frequently lose it again by 

 sharing it with the neighbouring molecules in the form of 

 kinetic energy, i.e., of thermal agitation. This is the com- 

 monest case, in which the absorption of light produces a 

 heating up of the absorbing substance; the energy of radiation 

 is thus put to the worst use, since it is converted into heat, a 

 degraded form of energy. 



Again, the molecules excited by hght may return to their 

 normal state by losing, in the form of light, the energy that 

 they have absorbed — this produces fluorescence and phos- 

 phorescence — or the energ>' of excitation may be transferred to 

 a single foreign molecule of another chemical nature by a 

 secondary impulse. This is a method of exciting molecules 

 which are not excited directly by light; they are mixed with 

 other molecules capable of absorbing this light. It is the 

 principle of the sensitization of photographic emulsions 

 containing silver bromide, by colorants, which enables the 

 sensibiUty to be extended into the red and the near infra-red, 

 while, without colorants, photographic emulsions are sensitive 

 only to the blue, the violet and the ultra-violet. 



The molecules, activated by Hght, may therefore, wholly 

 or in part, lose their excitation in several ways before they 

 have been able to take advantage of their state of activation 

 to react chemically. To a certain number of photons absorbed, 

 there corresponds a smaller number of molecules entering 

 into reaction. 



But the opposite also occurs — a photon may bring into 

 play a large number of molecules. The example just quoted, 

 of the action of chlorine on hydrogen, will illustrate this. 



According to Nemst's hypothesis (which has since been 

 revised) hght dissociates the molecule CI 2 into two CI atoms: 



CI2 — ^Cl+Cl (1) 



Each CI atom then acts chemically, without the inter- 

 vention of light, on the molecule of hydrogen : 



CI+H2— >HC1+H (2) 



