29 



prior to cooling by bubbling through N2. If the cooling is effected 

 in the presence of the absorbed atmospheric oxygen, the result is 

 the one shown in the next tube to the right. The O2 present made 

 no difference to the fluorescence of the unfrozen part, but made 

 the frozen part emit an orange phosphorescence. The O2 rendered 

 the triplet state unstable, making it emit light, "allowing" the ex- 

 cited electrons to return from their lower triplet state to the 

 ground state, giving off their excess energy in the form of photons. 



The last tube on the right contained besides riboflavine the 

 atmospheric O2 and 10"^ M potassium iodide. As the figure 

 shows, the phosphorescence was completely "quenched." Transi- 

 tions are accessible to regulatory influences both ways, as shown 

 by the opposite action of O2 and I'. If the one labilized the triplet 

 state and promoted the T -^ G transition, the other abolished it. 

 Such antagonistic effects can be produced by various substances. 

 Adrenaline, for instance, wipes out the weak phosphorescence of 

 frozen rhodamin in 10"^ M concentration while acetylcholine pro- 

 motes it and so the enhancing effect of .0025 M acetylcholine can 

 be compensated by 10"* AI adrenaline. Adrenaline quenches the 

 phosphorescence of riboflavine in 10"^ M concentration. 



The light emission of a watery riboflavine solution, frozen with- 

 out the previous elimination of oxygen, has been observed previ- 

 ously by Dhere and Castelli, and interpreted correctly as a phos- 

 phorescence. 



We can thus sum up by saying that water has brought about a 

 profound change in the excitational states of the two fluorescent 

 substances studied, making forbidden transitions into probable 

 ones, and the question arises whether the behavior of rhodamin 

 and riboflavine represent some general rule or whether we have 

 stumbled here on two substances that behave peculiarly. As far as 

 my experience goes we are faced here with a general rule. All 

 fluorescent substances tested showed such an extraordinary be- 

 havior at low temperatures in water. The following examples may 

 be quoted: on freezing of its watery solution in dry ice, the green- 



