26 STUDIES IN LUMINESCENCE. 



HISTORICAL. 



In 1888 B.Walter, 1 using the Vierordt spectrophotometer, found the ratio 

 of the fluorescent light emitted to total incident light absorbed, Fl/ A, to 

 increase with dilution. This law had been previously conjectured by 

 Lommel, 2 whose mathematical theory Walter 3 used in verifying his own 

 results. In connection with this work Walter tested Beer's law of absorp- 

 tion for fluorescein by four measurements. He found the law to hold true 

 for dilute but not for concentrated solutions. 



Walter briefly states his conclusions as follows: 



"Ability to excite fluorescence, Fl' A, in the most concentrated solutions, 

 is infinitely small or zero. After Fl/A obtains a measurable value it 

 increases in proportion to dilution to a certain dilution called the 'critical 

 point.' For greater dilutions Fl/A remains constant." 



In explanation of the above, Walter 4 advances a molecular group theory 

 which is of interest, preceding as it does the present theory of ionization 

 given by Buckingham. 5 Walter summarizes his theory in the following 

 statements : 



1. Every separate molecule of a given substance in solution absorbs, 

 so long as it remains in the separate condition, the same fraction of the light 

 falling upon it, no matter how great its distance from other molecules in the 

 solution may be. 



2. Every separate molecule of a given fluorescent substance in solution, 

 so long as it is in the separate condition, changes the same fraction of 

 absorbed light into fluorescent light, no matter how great its distance from 

 neighboring molecules may be. 



3. As soon as molecules begin to combine in groups the validity of these 

 statements ceases. Fluorescence entirely ceases in such a group and 

 absorption extends over wave-lengths which a separate molecule is not able 

 to absorb. 



Walter's idea of the separate molecule seems to correspond closely with 

 what is now known as the ion. 



In 1894 E. Buckingham performed a series of careful experiments to 

 discover whether or not fluorescent substances are in a state of ionization. 

 His investigations led to the conclusion that, at least in some cases, fluo- 

 rescence is due to ionization and produced only by that part of a solution 

 which is ionized. Certain ions, along with their other well-known optical 

 properties, possess this property of fluorescence. 



The conclusions drawn from the spectrophotometric work described in 

 Chapter I are found to apply also to resorufm. For example: 



1. The characteristic fluorescence band is situated near the edge of the 

 absorption band and is steeper on the side toward the violet. 



2. As fluorescent light passes through greater thicknesses of liquid, the 

 position of the maximum in the fluorescence spectrum is shifted toward 

 the red. 



>B. Walter. Wied. Ann., 34, 1888. 6 E. Buckingham, Zeitschrift fiir physikaliscbe 

 -Lommel, Poggendorff's Ann., 160, p. 76, 1877. Chemie, 14, p. 129, 1894. 



3 B. Walter, Wied. Ann., 36, p. 502, 1889. 6 E. Buckingham, /. c. 

 B. Walter, Wied. Ann., 36, p. 518, 1889. 



