Fluorescence and Arrangement of Molecules, 477 



that the absorbing power of fluorescein also showed itself 

 constant throughout this range, whereas in stronger solutions 

 this was no longer the case. What is the cause then of the 

 irregularities in the fluorescibility and absorption in these 

 latter solutions? The second of the results stated above 

 points to the explanation. For since, according to it, the 

 fluorescibility alters in the same manner for all rays producing 

 the fluorescence, the decrease in this quantity in concentrated 

 solutions could hardly be produced by any mere weakening 

 of these waves (such as, for example, might be explained 

 by the fluorescein molecules being too crowded), for then 

 different wave-lengths would necessarily produce some 

 difference in the effect ; the explanation is rather to be sought 

 in the decrease in the number of those molecules by which 

 the fluorescence is set up. Fluorescein and similar com- 

 pounds must according to this view exist in solutions of 

 different degrees of concentration in at least two molecular 

 conditions, a fluorescing and a non-fluorescing one ; and 

 indeed this hypothesis was soon placed beyond a doubt by 

 the discovery of a very remarkable phenomenon of fluorescence. 

 Before I describe this phenomenon I will, for the sake 

 of ease in referring to them, distinguish three grades of 

 solution of the bodies under consideration : — 



(1) Those in which only non-fluorescent groups of mole- 

 cules exist (Group solutions). 



(2) Those in which these are gradually disintegrated and 

 pass into smaller fluorescing molecules (Transition solutions). 



(3) Those in which this transition is completed (Perfect 

 solutions). 



In perfect solutions we have, according to the above, to 

 deal only with molecules all having the same properties, which 

 for shortness I shall call " single " molecules, and for which 

 this law holds good : — Every single molecule throughout the 

 whole range of dilution in which it preserves its single con- 

 dition absorbs always the same fraction of the quantity of light 

 falling upon it, and converts always the same fraction of the 

 absorbed energy into fluorescent light. But as soon as the 

 single molecules begin to arrange themselves in groups, as 

 is the case in transition solutions, the absorption becomes 

 quite irregular, and in the group the property of fluorescence 

 is entirely lost. 



These and all statements in the latter part of this paper are 

 based upon the following phenomena. If a spectrum was 

 allowed to fall on a perfect solution of fluorescein or Magdala 

 red, one saw that each ray absorbed by it gave rise to a 

 corresponding quantity of fluorescent light ; in transition 



Phil. Mag. S. 5. Vol. 28. No. 175. Dec. 1889. 2 N 



