Study of the Fluorescence of Iodine Vapour. 533 



tube, and measure the intensity of! the very feeble illumi- 

 nation o£ the background. This constant quantity was 

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 fluorescence. 



When working above room-temperature, the entire tube 

 was immersed in a rectangular glass tank filled with distilled 

 water at the desired temperature. 



The variation of the intensity of the fluorescence as a 

 function of temperature is shown by curve A, fig. 1, 



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ordinates representing intensities and abscissae temperatures. 

 It is to be understood, of course, that the alteration of the 

 temperature influences the intensity of the fluorescence only 

 •by changing the vapour-density. 



The variation of the density of the iodine vapour with the 

 temperature is shown by curve B, the pressures in millimetres 

 of mercury (ordinates) being indicated to the left of the 

 •curve. This curve was plotted from the values obtained by 

 Baxter, Hickey, and Holmes (J. American Chemical Soc. 

 xxix. p. 127, Feb. 1907) between 0° C. and 55° C. They 

 ;give no values below 0°, though a faint fluorescence can be 

 •observed at —30°, to which point I have carried the curve 

 hj rough exterpolation. 



