Destruction of Fluorescence of Iodine and Bromine Vapour, 309 



Let d = d r when fi — l. 



Then evidently d=fid f . 



For glass lenses of 10 cm. focus, d — 20 cm., d' =21 , 

 when ^ = 4/3. Therefore it is quite possible to measure both 

 of these lengths to a high degree of accuracy. In the 

 apparatus used, the two lenses are fitted on the ends of two 

 tubes sliding one within the other through a stuffing-box. 

 The apparatus is mounted on the table of a spectrometer 

 whose telescope and collimator have been previously focussed 

 for parallel light. The distance between the lenses is ad- 

 justed so that a clear image of the collimator slit is observed 

 in the telescope with the tube (a) full of air, (b) full of water 

 or other liquid. Index marks are placed on the tubes and 

 the distance between these may be measured with vernier 

 calipers, or by means of a travelling microscope if great 

 accuracy is desired. 



XXXVII. The Destruction of the Fluorescence of Iodine and 

 Bromine Vapour by other Gases. By R. W. Wood *. 



[Plate III.] 



AN extended study of the fluorescence of sodium, 

 potassium, mercury and iodine vapour lias shown 

 that the intensity of the emitted light is greatly reduced if 

 air, or some other chemically inert gas, is present. A quan- 

 titative study of the phenomenon, showing the relation 

 between the intensity of the fluorescence and the pressure 

 and molecular weight of the foreign gas, is much to be 

 desired as a means of testing any hypothesis which may be 

 made regarding the action of the gas upon the radiating 

 molecules. The vapour of iodine is especially suited to the 

 work, since its fluorescence can be observed at room tempe- 

 rature in glass bulbs, and the conditions of pressure, density, 

 &c. can be accurately determined, which is nearly or quite 

 impossible with sodium vapour. 



A satisfactory theory of the phenomenon should not only 

 explain the destruction of the fluorescence by the inert gas, 

 but also the failure of bromine to show any trace of fluor- 

 escence when under the same conditions as iodine vapour. 

 Its absorption spectrum is very similar, and yet it usually 

 remains quite dark even under the most powerful excitation. 



Some years ago I suggested the hypothesis that the 

 molecule might be capable of storing up a certain amount of 



* Communicated by the Author. 



