Mr. H. Jackson on Phosphorescence. 403 



and to fail to get some evolution of light. Phosphorus, 

 sodium and potassium, ether, many aldehydes, and a host of 

 organic compounds may be cited as instances. The experi- 

 mental illustrations of these are not, however, suited to an 

 audience of more than a very few. The same may be said 

 of the examples of animal and vegetable phosphorescence. 

 It is proposed therefore to deal more especially with the 

 second class, and to limit the experiments to the cases where 

 the light given out is visible and not of such a character as 

 to necessitate the use of a photographic plate. This evolu- 

 tion of light may occur in varying conditions. In instances 

 such as solutions of quinine and fluorescein and many solids, 

 of which thallene is a good example, the duration of the 

 phosphorescence is so short that it may be said to last only 

 while light is acting upon them. Balmain's luminous paint 

 is an illustration of the persistence of the phosphorescent 

 light. With many minerals, notably some fluorspars and 

 felspars, light is given out when they are slightly heated or 

 in some cases only crushed. 



The most brilliant phenomena are those which can be 

 studied when many bodies are excited with electric dis- 

 charges inside a Crookes's vacuum-tube, while outside of a 

 slight modification of his focus-tube fairly brilliant phos- 

 phorescence can be obtained by the action of Rontgen rays 

 upon several substances — notably upon some of the platino- 

 cyanides. 



In dealing with the whole subject of phosphorescence with 

 the view of attempting to connect all the various phenomena 

 together, it is convenient to divide it into the nature of the 

 substance giving out the light, the nature of the light given 

 out, and the nature of the exciting causes. 



With regard to the nature of the substance either very 

 much or little might be said : very much from the details of 

 numerous experiments with a great number of compounds ; 

 but little from the point of view of general principle. The 

 most important question in this respect is probably the ques- 

 tion of the relation of phosphorescence to the purity of the 

 substance giving out the light. Experiments with carefully 

 prepared compounds of many metals make it clear that not a 

 few substances can be made to exhibit phosphorescence when 

 they are so free from impurities that none can be detected by 

 any analytical methods. In some cases, however, there is 

 either no light given out under any of the conditions for 

 exciting phosphorescence, or the light is so feeble that it is 

 necessary to add impurities so as to obtain a suitable mole- 

 cular condition for rendering a substance responsive to excite-- 



