258 THE ROYAL SOCIETY OF CANADA 



An account of an investigation of the destruction of the 

 fluorescence of solutions has recently been published by Wood/ who 

 found that a fluorescent solution was generally transformed on ex- 

 posure to sunlight to a coloured non-fluorescent liquid with a different 

 absorption band. Continued exposure rendered the solution colour- 

 less. He has also shown that a fluorescent solution (rhodamine) may 

 be bleached although prevented from fluorescing by maintaining at a 

 temperature of 100°C. This would indicate that a transformation 

 of the solution may not be as intimately connected with its fluor- 

 escence as supposed by Perrin. 



It was thought to be of interest to make a photometric study of 

 the rate of destruction of fluorescence during exposure to the exciting 

 light. 



II. Apparatus and Method 



An aqueous solution of aesculin (CisHieOg) was exposed in a test 

 tube to the light of a mercury lamp. At regular intervals the solution 

 was removed and its fluorescence compared with that of the original 

 unexposed solution. 



The quartz mercury lamp used to cause the solution to decay 

 was 75 cms. in length and carried about 10 amperes. Although the 

 light emitted by it was very intense the solution suffered only a slight 

 rise in temperature, which could be neglected, as cooling took place 

 at the intervals for testing the fluorescence. 



Fig. 1 shows the arrangement of the apparatus for measuring the 

 intensity of fluorescence for which a Nutting spectro-photometer was 

 used. The light from a self -regulating carbon arc {A) was reflected 

 downwards by a mirror into the two cells containing the fluorescent 

 solutions to be compared. The fluorescent light from the cells passed 

 through a stop {F) and entered the openings of the photometer, in a 

 direction at right angles to the final direction of the exciting beam. 



By varying the height of the stop the fluorescence at any depth 

 of liquid could be measured. 



In the experiments described below a stop 4 mm. in width was 

 used and the intensity of the first 4 mm. of the liquid measured. 

 The solutions used were sufficiently dilute to show no absorption and 

 thus give a uniform image in the photometer. 



A screen at G served as a black background for the fluorescent 

 light and prevented the light of the arc from entering the photometer. 



7R. W. Wood, Phil. Mag., pp. 757-765, 1922. 



