Mr. H. Jackson on Phosphorescence, -107 



low vacuum, or of no vacuum at all, the charged particles 

 would discharge themselves against the intervening gas, which 

 would in its turn respond to the rapid oscillation, and give out 

 its own particular coloured light. The expression u short 

 aethereal waves " is used intentionally; for if there should be 

 forthcoming experimental evidence of the complex molecular 

 structure of a gas, it is reasonable to suppose that in a high 

 vacuum, with consequently a high potential at the electrode, 

 the internal electrical repulsion in a group would tend to a 

 dissociation resulting finally in the simplest form of system 

 capable of separate existence in those conditions. It might 

 be expected that the oscillation -frequency of so simple a 

 system would be very high. 



Here it may be stated that this comes to practically the 

 same thing as Sir William Crookes's original conception of 

 radiant matter. 



Leaving the method of electrical excitation in vacuo for 

 obtaining phosphorescence, we may now turn to light as a 

 source of oscillations. For the sake of simplicity, it will be 

 best to continue the experiments with the same substance, 

 viz. lime. If this body be exposed to the light of the sun, of 

 the electric arc, of a hydrogen flame, and of a great many 

 other substances in a state of vigorous combustion, a phospho- 

 rescent effect is obtained, feeble in comparison with the results 

 in vacuo, but apparently similar in kind. The best light for 

 inducing the phosphorescence is the spark from a fairly 

 powerful coil with a ley den-jar in circuit. Many specimens 

 of lime go on giving out light for a considerable time after 

 exposure. A cylinder of lime such as is used in the pro- 

 duction of the lime-light glows quite visibly when it is rotated 

 before a jar-spark. 



The light from the sun is not so active in inducing this 

 glow; but with suitable arrangements a fairly visible result 

 can be obtained. The colour of the glow from most lime 

 made from limestones is an orange-red becoming a golden 

 orange when the lime is heated. The introduction of glass, 

 mica, or Iceland-spar between the spark and the lime cuts 

 off the glow at once, since these bodies are opaque to the 

 undulations to which lime of this kind responds. Quartz, 

 rock-salt, and selenite are quite transparent. 



It is found that the different forms of lime which have 

 already been exhibited in vacuum-tubes yield, when exposed 

 to the jar-spark, their specially coloured phosphorescent glows. 

 But these are difficult to see ; they are very faint when pure 

 specimens of lime are used. However, there is a way out of 

 the difficulty. The faint light scarcely visible at the ordinary 



