764 Prof. R, W. Wood on 



hot solution, yet decomposition proceeded at practically 

 the same rate in the two cases. At the end of half an hour 

 the colour had been considerably reduced, but no difference 

 could be detected, except that the fluorescence of the high- 

 temperature tube, after it had been cooled off, was slightly 

 greenish in colour, while that of the other was orange- 

 yellow as at the beginning of the experiment, only much 

 less bright owing to the partial decomposition. 



This was a rather disappointing result from the point of 

 view of confirming Perrin's theory. A control experiment 

 showed that the rhodamine solution was not bleached by 

 an hour's heating in a sealed tube immersed in a beaker of 

 boiling water. 



Attention is drawn to the fact that not all samples of 

 rhodamine are sensitive to temperature. I have, in all, 

 about a dozen different rhodamines of which only two show 

 the very marked loss of fluorescent power at 100°. 



Function of the Intensity of the Exciting Light. 



A very remarkable relation between the rate of breakdown 

 of eosine and the intensity of the exciting light was dis- 

 covered, which has probably a very direct bearing on 

 Perrin's theory. Stating it in simple form, we may say 

 that high intensities acting for a short time produce a 

 greater amount of decomposition than low intensities acting 

 for a long time, the time being increased in proportion to 

 the reduction of the intensity, of course. This means that 

 the change is not proportional to the amount of energy 

 extracted from the incident light — a rather surprising result, 

 and one at variance with the Bunsen-Poscoe law. 



The experiment was made in the following way : — Two 

 circular cells were made, each six inches in diameter and 

 J inch in thickness, by cementing glass disks to brass 

 rings. Each cell was filled with a solution of eosine of the 

 same concentration. One cell was exposed normally to 

 direct sunlight, the other to sunlight concentrated by a six- 

 inch lens. The solar image was formed near the bottom of 

 the cell, so that the convection would be as complete as 

 possible, Ln each case the same amount of light is passed 

 through the cell, and through, the same thickness of solution 

 (J inch). The only difference is that in one case it is 

 spread uniformly over the whole area of the cell, and in the 

 other case it is concentrated on an area of very small size. 



