PAPERS ON CHEMISTRY AND PHYSICS 271 



TVe are familiar with the use of the same principle in the 

 spark-plug tester containing neon gas, 



So far only luminescence produced by thermal or elec- 

 trical means has been considered. Some other light- 

 producing phenomena will now be indicated. 



TVhen light strikes a body it may be in part transmit- 

 ted, reflected or absorbed. That which is absorbed may 

 be re-radiated in longer wave lengths. This is called 

 fluorescence. The li^rht enersy may actually be stored 

 and emitted later — which is called phosphorescence. 

 The only distinction between these two phenomena is that 

 phosphorescence may continue after the incident beam is 

 cut off while fluorescence does not. In some cases the 

 incident beam is of longer wave length than the emitted 

 beam. Here the vibration seems to have been stepped- 

 up to a higher frequency. This phenomenon is known 

 as caloreseen-ce. 



Phosphorescence and fluorescence in substances is 

 thought to be due to minute traces of impurities. Per- 

 fectly pure substances are incapable of phosphorescence. 

 A certain proportion of the impurity gives a maximum 

 effect. Thus phosphorescence is governed by the follow- 

 ing conditions : 



1. The amount of the impurity present. 



2. The nature of the impurity. 



3. The temperature of the substance. 



4. The intensity and duraton of the light stimulus. 



Ultra-violet light is more active in producing phos- 

 phorescence and fluorescence than is ordinary light. 

 Ultra-violet light can thus be converted into visible light. 

 The mercury arc and the magnetic arc aive a light rich 

 in ultra-violet. Quartz is used rather than srlass for the 

 mercury arc because its high meltinu* point (1700° C.) 

 allows a higher temperature for the arc and hence a bet- 

 ter efficiency, and also because quartz allows a thousand 

 times as much ultra violet light to filter through- as does 

 glass. All radiations of wave length less than 3300 A 

 are absorbed by glass. Quartz is permeable to light of 

 wave length above 1850 A. 



