310 Prof. R. W. Wood on Destruction of the Fluorescence 



energy without the emission of light, but that a saturation 

 point must be reached event ually, after which there will be 

 an emission of radiation. If we assume that on collision with 

 another molecule the energy absorbed by the molecule is 

 transformed into heat, the internal energy dropping back to 

 its original value, and the molecular velocity increasing in 

 proportion, it is clear that if the mean free path is traversed 

 before the saturation point is reached, there will be no fluor- 

 escence. On this hypothesis we should explain the failure of 

 bromine to fluoresce by ascribing to the bromine molecule a 

 greater capacity for storing energy. In other words, the 

 path cannot be increased sufficiently to allow the saturation 

 point to be reached before a collision occurs. It seemed 

 possible to test this theory by experiment. By sufficiently 

 increasing the length of free path, we ought to be able to 

 observe fluorescence, provided that a sufficient number of 

 molecules remain to produce a visible illumination. A small 

 amount of bromine vapour was introduced into a bulb, and 

 condensed upon the wail by the application of solid carbon 

 dioxide and ether. The bulb was then exhausted to the 

 highest possible degree and sealed. On warming it to room 

 temperature the bromine vaporized, and though it was so 

 highly rarefied that it showed no colour, no fluorescence 

 could be detected. 



Sunlight was now concentrated at the centre of the bulb 

 by means of a portrait lens having a ratio of focus to aperture 

 of 2*3. Even in a dark room with careful screening off 

 of diffused light, no fluorescence could be detected. The 

 outside of the bulb was now touched with a piece of solid 

 carbon dioxide, which gradually condensed the bromine upon 

 the wall. In two or three seconds a faint green fluorescence 

 appeared, which vanished almost immediately, owing to the 

 complete removal of the bromine vapour. There appears 

 then to be one density at which bromine shows a visible 

 fluorescence. At higher densities collisions destroy it, at 

 lower, there are too few molecules present. This appears to 

 be in accord with our hypothesis regarding absorption of 

 energy, saturation point, &c. ; but more recent work, made 

 in collaboration with J. Franck, has shown that another factor 

 comes into play. The question will be considered again in 

 the paper immediately following the present one. 



The method of observing the iodine fluorescence has been 

 so improved that it is now possible to demonstrate it to the 

 largest audience. A large bulb 15 or 20 cms. in diameter 

 is prepared by drawing down the neck of a round-bottomed 

 flask, which should be most carefully cleaned with aqua regia 



