15 



empty ones, then it will be an insulator, a dielectric. However, if 

 the distance between tlie highest filled band and lowest unfilled 

 band is not too great, then electrons may be raised from the former 

 into the latter and make the substance conductant. If the distance 

 between the two levels is small, even thermal agitation may suffice 

 to produce this change, in which case, the substance is a "semicon- 

 ductor." Absorbed photons may do likewise leading to "photo- 

 conductivity." I have pointed out earlier (1946, 1947) that it is 

 possible that proteins may have an electronic structure analogous 

 to that of semiconductors. Evans and Gergely found that the 

 hydrogen bonds, within the protein molecule, may create con- 

 ditions which could lead to the development of such continuous 

 bands, though their results were not quite conclusive. The distance 

 between the highest filled and lowest unfilled band was found to 

 be rather great, energies of the order of 100,000 calories being 

 needed to raise an electron from the first to the second. 



Experimental evidence for the existence of a semiconductor 

 nature of biological material is not missing. Eley, Parfitt, Perry, 

 and Taysum found semiconductivity in plasma albumin, fibrinogen, 

 and edestine. The gap between the highest filled and lowest un- 

 filled band was somewhat smaller than calculated by Evans and 

 Gergely and water was found to decrease the energies needed to 

 lift electrons over it. 



W. Arnold (personal communication) showed recently that 

 chloroplasts can "store light," that is, conser^^e the energy of ab- 

 sorbed photons, which energy they could again shoot out later in 

 the form of light, if heated. This could hardly be explained any 

 other way than by assuming the existence of energy bands. The 

 electrons raised to the higher band seem to have been trapped 

 there in "wells," from which they can be chased out by heat agita- 

 tion. The differences in the energy of the bands were found to be 

 still lower than found by Eley et al., bringing the energies needed 

 for raising electrons to the higher empty level into the realm of 

 biological possibilities. 



