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paramagnetic molecules, as Oj, or of heavy atoms with a high 

 atomic number, as iodide, the nucleus of which can create an 

 electromagnetic disturbance in its vicinity. If an excited electron 

 enters it, it may reverse its spin. A beautiful example of this has 

 been given by Kasha (1952), who showed that the almost pure 

 singlet spectrum of dichloronaphthalene went over into an almost 

 pure triplet spectrum if he replaced the CI by the heavier iodine. 

 But what is even more important — it was not necessary to intro- 

 duce the iodine into the molecule in order to produce this change. 

 It was sufficient to add it to the solution in the form of ethyliodide. 

 These effects seem to me especially fascinating because iodine is 

 contained in one of the main regulators of cellular energetics, 

 thyroxine (and related compounds), while O2, which is one of the 

 very few molecules paramagnetic in its normal state, is most inti- 

 mately involved in the energetics of life. 



The biological energy unit, the energy of the ^P is of the order 

 of 10 Calories, which corresponds to a wavelength of 2-3ft, which 

 is to say that a photon of this wavelength has the same energy as a 

 '—P. This wavelength corresponds to the near infrared. It is thus 

 this spectral region which will have the greatest direct interest for 

 the biologists. It also has an interest of its own. It is here that pure 

 electronic and vibrational excitations meet and transition between 

 the two is the easiest (which might be one of the reasons why the 

 biological quant is located here). Similarly to other borderlines 

 this spectral region, too, is a no man's land, lying beyond the 

 domain of current spectroscopy and on this side of the professional 

 infrared. This region may hold surprises even for the physicists. 

 Its secrets are also guarded by technical difficulties, as the lack of 

 good detectors, and water which has several absorption bands in 

 this region. 



