476 B. Walter on the Relation between 



obtained with sufficient accuracy by the formula 



F 



/= 



1—aot 



sec. 30° f 



in which F is the intensity of the fluorescent light from G 2 , 

 measured as stated above ; and the fractions a and a denote 

 the " coefficients of transmission " (Langley) of the fluorescent 

 light emerging from Grj for the film of liquid in G 2 . They 

 therefore express in what ratio the light incident on G 2 is 

 weakened on emergence again. These coefficients were like- 

 wise determined by the Vierordt spectrophotometer, and thus 

 all the data requisite for the calculation of the fluorescibility 

 were obtained. 



The following results were obtained : — 



(1) The fluorescibility of very concentrated solutions of 

 fluorescein (from 40 per cent, down to about 3 per cent, of 

 salt) is zero, or at any rate very small ; from this point it 

 suddenly increases very quickly as the solution is rendered 

 more dilute, the rate of increase being at first fairly uniform. 

 But from 5 per cent, downwards the increase becomes more 

 and more slow, and it ceases entirely when the solution con- 

 tains 0*02 per cent, of salt ; so that the fluorescibility remains 

 constant from this point onwards to the most dilute solutions, 

 the observations extending as far as a OvOOOOOl per cent, 

 solution. 



(2) The alterations of the fluorescibility remain the same 

 whatever be the wave-length of the rays used to excite it ; 

 in the experiments rays were used both below and above 

 those giving the maximum of absorption. 



Alcoholic solutions of Magdala red of different degrees of 

 concentration showed the same fluctuations of fluorescibility, 

 except that in this case no solution could be obtained so con- 

 centrated that the fluorescibility became absolutely zero. 



II. Tlieoretical Deductions. 



The fact that the fluorescibility remains constant in the 

 large number of solutions having a greater dilution than 0*02 

 per cent, must be regarded as the most important of the 

 results just stated ; for this means that under these circum- 

 stances the same quantity of substance always gives out the 

 same quantity of fluorescent light. This can lead to no other 

 theoretical conclusion than that the particles of fluorescein, 

 which give rise to the light, preserve the same constitution 

 unaltered throughout the whole of the range of dilution con- 

 sidered; and this conclusion agrees very well with the fact 



