410 Mr. H. Jackson on Phosphorescence. 



paint, and even less in the limes which require heating to 

 show up their phosphorescence, while in the case of the 

 chlorophane and many other minerals, the condition of strain, 

 however set up, can apparently be retained indefinitely. 

 Specimens of lime, after exposure to the jar-spark, have been 

 found to give out light when heated after being four years 

 in the dark. It seems not altogether improbable that the 

 influence of impurities in promoting phosphorescence may 

 often be attributed to their interfering with the freedom of 

 movement and so permitting the groupings of the substance 

 to be sufficiently highly charged. The effect of heat in 

 rendering a substance a better conductor can be well studied 

 with pure substances in vacuo under the electric discharge. 



Under the vigorous bombardment of radiant matter the 

 temperature of the substance rises. In some substances this 

 leads to an increase in the brilliancy of the glow maintained 

 often even when the heating is very considerable ; in others 

 the hotter portions are marked out by a complete absence of 

 phosphorescence. Observation seems to favour the conjecture 

 that this absence is in many cases to be explained on the 

 hypothesis that the heat endows the molecules with such free- 

 dom as to practically render them uninsulated. To pursue 

 this part of the subject any further would lead to a discussion 

 of a question that can only be referred to. It is the con- 

 sideration of how far the change of glow in some specimens 

 of lime from a red or orange colour in a low vacuum to a green 

 or blue glow in a high vacuum is to be attributed to shorter 

 oscillations in the exciting cause, and how far the change is 

 connected with a dissociation of complex groupings into 

 simpler ones ; a dissociation which may be considered to be 

 brought about by the rapid oscillations breaking up the lime 

 groups into two or more smaller groups. Connected with this 

 is also the question dealing with the possibility of phospho- 

 rescence being coincident with the recombination of the 

 separated smaller groupings ; but this part of the subject can 

 only be illustrated by experiments of too minute a character 

 to be suitable to a lecture, and involves besides the study of 

 too many details. One other thing which must be taken into 

 account in drawing any deductions from the change in the 

 colour of the glow as the temperature rises is that in some 

 cases the effect of heat is to discharge some colours in a 

 complicated substance, and so leave visible others which were 

 before masked. 



The whole question of the inter-relations of the molecular 

 weights of the phosphorescent substances, of the wave-lengths, 

 of the exciting undulations, and of the wave-lengths of the 



