Mr. H. Jackson on Phosphorescence. 409 



give an orange glow in a moderate vacuum while a portion 

 of the same specimen is exhibiting a blue glow in a high 

 vacuum. The readiness with which this blue glow appears 

 and the time which it takes to develop must be taken into 

 account in dealing with its supposed origin and with its 

 relevancy with the question of the relation of the rapidity of 

 the exciting undulation to the wave-length, i. e. to the colour, 

 of the phosphorescent light. Perhaps it is advisable to leave 

 this point for the moment and to turn to the second con- 

 sideration. This deals with the question of the duration of 

 the phosphorescence. 



At the beginning it was shown that some bodies glow only 

 while light is acting upon them, or while they are under 

 the direct influence of an electric discharge. In others 

 there was a marked after-glow ; while still others required 

 the application of heat before any phosphorescence was visible, 

 or, as in the case of the limes, before the phosphorescence 

 was easily visible. With Balmain's luminous paint, or with 

 anybody which gives a marked phosphorescence that lasts for 

 some time after withdrawal from the exciting influence, it can 

 be readily shown that lowering the temperature reduces the 

 brilliancy of the glow but lengthens the time during which 

 it lasts. The effect of heat has already been mentioned as 

 vastly increasing the brilliancy ; but it greatly diminishes 

 the duration of the light. On the other hand Professor Dewar 

 has shown that great reduction of the temperature will cause 

 the phosphorescence to linger for a considerable time in many 

 substances which had hitherto been considered as practically 

 non-phosphorescent. The different behaviours of substances 

 in this respect can perhaps be best brought under one explana- 

 tion by applying the idea of a statical charge or a condition of 

 strain to the phosphorescent substances themselves. Duration 

 of phosphorescence would then be a measure of rapiditv of 

 discharge, If it be supposed that, the strain having been 

 set up in the particles of a substance, these discharge them- 

 selves against one another, or rather against uncharged par- 

 ticles, then a substance with great freedom of interchanging 

 movement among its particles would fail to show any sign 

 of phosphorescence ; since the strain would be released or 

 conducted away by rapid transference before a condition could 

 be set up out of which oscillations of sufficient amplitude could 

 arise. With rather less freedom of movement among the 

 particles the non-conducting state might be reached by 

 restricting the extent of that movement by cold, as in 

 Professor Dewar's experiment. Still less freedom of inter- 

 change may be considered to obtain in Balmain's luminous 



