The Nineteenth Century 247 



each quantum of light absorbed may cause one molecule to react in 

 a primary photochemical reaction. Conversely, chemiluminescence 

 can only occur if the free energy of the reaction is sufficiently high 

 to account for the quantum energy of the frequency emitted when 

 one molecule undergoes change. ^^ Based on a mole of material 

 undergoing chemical change, the energy necessary (a quantum, hv, 

 multiplied by the number of molecules in a gram-mol, called an 

 einstein) is rather high. A blue luminescence requires a free energy 

 change of some 60,000 calories. Thus the modern student of bio- 

 luminescence, concerned with the mechanism of light production in 

 any organism, must answer this fundamental question— what reac- 

 tion produces sufficient energy to result in frequencies observed in 

 the bioluminescence spectrum? Only then can the emission of light 

 be understood. Such a question was unheard of before 1900. With 

 this new outlook, research on all types of luminescence progressed 

 rapidly during the twentieth century, adding new names and new 

 theories to history. 



^' The relation of quantum energy in the light emitted to the heat of reaction was 

 first considered by F. Haber and W. Zisch {Ztschr. fur Physik. 1: 302-326, 1922) in the 

 case of the yellow luminescence observed when chlorine and sodium vapor are mixed. 

 Free energy of reaction Af is a more correct designation, related to heat of reaction 

 AH by the expression, Af = AH — T^S, where T is absolute temperature and AS the 

 entropy of activation. 



