318 Fluorescence of Iodine and Mercury. 



and the fluorescence, caused by condensing the light of a 

 cadmium spark to a focus at the centre of the bulb, observed 

 at different temperatures. Small amounts of any foreign 

 gas were admitted by lowering the mercury below the 

 junction of the tube B, and allowing the gas to enter from a 

 reservoir through the tube B, the pressure being read with a 

 manometer. This pressure was about doubled by raising the 

 mercury to its original level. By raising the mercury to a 

 higher level still greater density of the gas can be obtained, 

 its pressure being easily determined by measuring the 

 difference of level of the mercury surfaces in the flask and in 

 the reservoir B, when the cock A is open. Closing the cock 

 seals off the bulb, and heating then causes no change of level. 



While oxygen at 3 mms. destroys the fluorescence, helium 

 at a pressure of 1 atmosphere scarcely affects the intensity 

 at all ! 



The fact previously observed by Wood, that the maximum 

 intensity of the fluorescence, in the absence of any foreign 

 gas, occurs at a pressure which is very different according to 

 the nature of the fluorescent gas, can probably be explained 

 by our hypothesis. In order to obtain a visible fluorescence 

 we must have a sufficient number of molecules present. Their 

 number must not, however, be so great as to cause them to 

 disturb each other. In a strongly electro-negative gas the 

 vibration electrons in one molecule are influenced by the 

 presence of neighbouring molecules. In the case of bromine, 

 therefore, which is more strongly electro-negative than iodine, 

 we have fluorescence only at very low pressures, probably 

 less than '001 mm., while in the case of iodine the maximum 

 intensity of the fluorescence occurs at a pressure of 

 about '2 mm. (tension at room temperature), and in the 

 case of the strongly electro-positive mercury vapour the 

 maximum intensity is not reached until we have a pressure 

 of several atmospheres. The intensity of the fluorescence 

 of mercury vapour at high pressures should be quantitatively 

 investigated. It will not be easy, for as the pressure 

 increases the fluorescence is confined to a layer of decreasing 

 thickness covering the wall through which the light enters 

 the bulb. 



Two facts seem to have been established by this investigation : 

 first, that the bound electrons, which emit the fluorescent 

 light, are effected in much the same way by the presence of 

 electro-negative gases as are the free electrons which carry 

 the current in vacuum tubes ; secondly, that the pressure at 

 which the maximum intensity of the fluorescence of a gas 

 occurs depends upon the electrical character of the molecule. 



