Fluorescence and Arrangement of Molecules. 479 



The fluorescent light of transition solutions of fluorescein and 

 eosine increases in intensity considerably as their temperature 

 is raised, doubtless because warm water decomposes the 

 groups of molecules of these substances more easily than cold 

 water. In Magdala red the contrast is still greater; cold 

 water cannot decompose its groups of molecules at all, but 

 warm water effects the decomposition fairly easily. That 

 these phenomena cannot be ascribed to an increased freedom 

 of vibration, produced by the application of heat, is proved by 

 the alcoholic transition solutions of Magdala red, since in 

 them the fluorescibility decreased slightly on heating. 

 According to what has been said above this was rather a 

 proof that warm alcohol does not dissolve Magdala red so 

 easily as cold; and indeed a cold saturated solution of it 

 became turbid on heating, an indication that the solid sub- 

 stance was being deposited again. 



A further circumstance which pointed to the existence in 

 transition solutions containing ammonium fluorescein of a 

 more complicated molecular arrangement than in perfect 

 solutions, was found in the fact that the former gave with 

 mineral acids an immediate dense precipitate ; the latter, on 

 the other hand, remained perfectly clear, and only after 

 several hours deposited fine crystalline needles of fluorescein 

 itself. Although it is generally not uncommon for a stronger 

 solution to yield a precipitate more quickly than a weaker 

 one under similar circumstances, yet the contrast is here so 

 great and the accompanying phenomena so remarkable that 

 one cannot refrain from bringing it into connexion with what 

 has already been stated with considerable certainty concern- 

 ing the differences in grouping of molecules. 



Finally, concerning the non-fluorescent group-solutions of 

 fluorescein, it was noted that they all possessed a surface 

 colour, which became stronger with increasing concentration 

 but whose quality remained exactly the same ; even the solid 

 body itself — of course not commercial acid fluorescein, but 

 ammonium fluorescein — possessed the same surface colour. 

 It is seen from this that fluorescein in its group-solutions 

 must still possess a stationary molecular condition, which 

 must be somewhat like that of the solid body. This theory 

 receives great support from the circumstance, proved in a 

 later contribution*, that the index of refraction of these 

 solutions increases in the same ratio as the percentage 

 composition. 



I cannot close this abstract without mentioning one obser- 



* Walter, Wied. Ann. xxxviii. p. 117 (1889). 

 2N2 



