of Iodine ami Bromine Vapour by other Gases. 313 



out influence upon the intensity o£ the emitted light. Oxygen 

 and C0 2 , on the other hand, reduced its intensity very rapidly 

 with increasing pressure. This I have provisionally ascribed 

 to " incipient chemical action " which of course in reality 

 means nothing at all. It was found that no permanent 

 chemical change took place, for on cooling the bulbs the 

 anthracene condensed upon the walls, and none of the oxygen 

 had disappeared. At high temperatures, however, the 

 oxygen acts upon the anthracene, and it seemed possible 

 that the first stage of the process might occur at the lower 

 temperatures used in the fluorescence experiments, the process 

 reversing as soon as the bulbs were cooled. This is what I 

 called "incipient chemical action." The probable real nature 

 of the action will be given in the subsequent paper. 



The only difficulty found in measuring the intensity of the 

 fluorescent light resulted from the slight change of colour 

 which occurred when the intensity was considerably reduced. 

 The colour always became slightly reddish at the higher 

 pressures, though it was only conspicuous when seen in the 

 photometer, the colour match not remaining perfect. 



The gases apparently weaken the green portion of the 

 fluorescent spectrum to a greater extent than the red. This 

 is a very interesting and important matter, which will be 

 discussed in the following paper. 



Experiments were also made with a highly exhausted bulb, 

 the vapour pressure of the iodine being varied by immersing 

 a side tube in freezing mixtures of various temperatures, or 

 heating the entire bulb in a water-bath. The values found 

 below room temperature are given in the following table, and 

 are shown in graphical form on Plate III. (small inset) : — 



Temp. 



Intensitj r . 





19 



43 



The relation is very 



G° 



29 



nearly linear. 



0° 



22 





-10° 



11 





-15 



6 





-20 



2 





Above room temperature no perceptible increase could be 

 detected. The intensity remained about the same up to 30°, 

 after which it gradually decreased. At high pressures the 

 fluorescence disappeared entirely 



It is probable that increased absorption compensates very 

 nearly for any increase that may occur above room tempe- 

 rature. Accurate measurements were impossible on account 

 of the change of colour due to the absorption of the iodine 

 vapour between the fluorescent cone and the eye. 



Phil. May. S. 6. Vol. 21. No. 123, March 1911. Y 



