LIGHT AND VEGETATION 105 



absolute zero; if we reckon temperatures from this zero, 

 which means adding 273 to the Centigrade temperature, we 

 obtain the absolute thermodynamic temperature, which is of 

 great theoretical interest. 



The average energy, E, attached to each degree of freedom 

 of a molecule, is proportional to the absolute thermodynamic 

 temperature, T; the coefficient of proportionahty, k, or 

 Boltzmann's constant, is the same for all the molecules, 

 whatever their constitution may be: 



E=CT 



where the constant k has the value 1-372x10— ^^ ergs per 

 degree C, so that E=4-ll x lO-i* ergs (at 300° absolute or 

 27° C. Trans.). 



A gram-molecule, which contains N true molecules 

 (N=6-06x 10^^), possesses, at ordinary temperature (about 

 300° absolute) and per degree of freedom, the following 

 thermal energy: 



NA:T=(6-061 x lO^^) x (1 -372 x lO-^^) x 300 

 =8-3 X 10^ X 300=24-9 x 10^ ergs. 



To employ a unit more familiar to chemists, this represents 

 an energy of 600 calories per gram-molecule. 



The average thermal energy is therefore kT, but the 

 energy at a given instant may be greater or smaller. In par- 

 ticular, a very small number of molecules acquire, at each 

 instant, an energy which is considerably higher than the 

 average and which may become sufficient to bring them to 

 the "activated" state; the lower the energy of activation and 

 the higher the temperature, the greater is this number. Thus 

 chemical reactions are in general accomplished more rapidly 

 when the combining bodies are heated. 



The bilUard ball analogy can be followed once more; 

 let us suppose that the table is shaken, the balls will be set in 

 disordered motion and from time to time one of them will 

 be impelled with enough force to clear the edge of the table. 

 A more vigorous agitation corresponds to a rise of temperature 

 and its obvious consequence is that the balls will fall at a 



