478 B. Walter on the Relation between 



solutions, on the contrary, a number of colours disappeared 

 by absorption in the liquid which gave no such light ; and 

 indeed the production of fluorescence took place over exactly the 

 same range of ivave-lengths as in the most concentrated of the 

 perfect solutions, while the absorption extended much further . 



It follows clearly that in transition solutions also it is only 

 the single molecules which produce the fluorescence, and that 

 the portion of light absorbed by these solutions which does 

 not reappear as fluorescent light is taken up by the groups of 

 molecules. The fluorescibility of a transition solution must 

 therefore decrease with increasing concentration for three 

 reasons : — (1) because the number of single fluorescing mole- 

 cules is constantly getting reduced ; (2) because the groups 

 of molecules take up an ever-increasing proportion of that 

 light which can produce fluorescence ; and (8) because the 

 groups of molecules exert a constantly increasing absorbent 

 action upon the fluorescent light which is formed by the 

 single molecules still remaining. This can be studied ex- 

 tremely well by observing the shrinking up of the band of 

 light at the less-refrangible end in the case of the spectrum of 

 fluorescent light, so that this alone affords a means of dis- 

 tinguishing at a single glance a perfect from a transition 

 solution. 



Stated more exactly, the three laws deducible from the 

 above phenomena are the following : — 



(1) In transition solutions there are present at the same 

 time both groups of molecules and single molecules ; and as 

 the dilution increases the latter multiply at the expense of the 

 former. 



(2) Only single molecules can give rise to fluorescent light, 

 not groups of molecules. 



(3) The range of absorption by single molecules extends 

 between quite definite wave-lengths ; groups of molecules on 

 the other hand absorb as a rule the neighbouring parts of the 

 spectrum as well. 



The latter phenomenon was most striking in the case of 

 fluorescein and Magdala red ; for while the absorption spec- 

 trum of a very thick layer of a perfect solution ended quite 

 suddenly in the red with a sharp boundary, the absorption of 

 a transition solution, even when the absorbing layer con- 

 tained altogether less material, stretched beyond this boundary 

 and was gradually lost on the other side of it (between it and 

 the ultra-red). In the case of eosine no such difference was 

 found, so that in this compound the group absorbs hardly 

 any more rays than the single molecule. 



These ideas were supported by another series of phenomena. 



