Fluorescence and Photo- Chemistry. 763 



rhodamine nearly ceases at 100°. Moreover, fluorescence 

 ceases if the concentration becomes too great, due to what 

 Perrin calls protective action. 



If, now, it be found that preventing the fluorescence by 

 these means prevents the bleaching of the solution by the 

 action of light, it will be a strong argument for the theory 

 of Perrin. On the other hand, if it be found that bleaching 

 proceeds at the same rate as before, we shall have grounds 

 for suspecting that the fluorescence is not directly connected 

 with the breakdown of the molecule. 



Both of these experiments have been tried. A strong- 

 solution of eosine w r as made which showed no trace of 

 fluorescence, even in concentrated sunlight. A carefully 

 measured amount of this solution (\ c.c.) was introduced 

 into a small cylindrical flat-bottomed bottle and illuminated 

 by a vertical beam of sunlight concentrated by the six-inch 

 lens for half an hour. The bottle was kept cool by immer- 

 sion in a beaker of water during the experiment. A similar 

 amount of the strong solution diluted with 5 c.c. of water 

 was subjected to a similar treatment for the same length of 

 time, fluorescing brilliantly throughout the experiment. The 

 contents of the first tube were now diluted to the same volume, 

 and the contents of the two tubes compared. The concen- 

 trated solution had suffered no change from the action of the 

 light, while the diluted solution was largely made up of 

 the photo-eosine, as was shown by its red colour and feebler 

 fluorescence. 



This indicates that proximity of the molecules protects the 

 solution against decomposition as well as against fluorescence, 

 and argues in favour of the theory that the decomposition 

 results from the fluorescence. 



The experiment of inhibiting the fluorescence of rhodamine 

 by raising it to a temperature of 100° was next tried. Two 

 small tubes were prepared, of equal size, and equal amounts 

 of a dilute solution of rhodamine introduced into them. 

 They were then sealed in a flame as close as possible to the 

 surface of the solution. One tube was mounted in a beaker 

 of cold water close to the wall, the other in a beaker of water 

 kept boiling by a Bunsen burner. Both were illuminated 

 simultaneously by sunlight concentrated by two six-inch 

 lenses, which formed images of the sun of a diameter about 

 half that of the tubes. In this experiment the conditions 

 were exactly the same, except that one tube was at a higher 

 temperature than the other. The fluorescence of the cold 

 solution was about twenty times as bright as that of the 



