214 Professor Silvanus P. Thompson [May 8, 



in a Crookes tube. There is the crystallo-luminescence of certain 

 materials when they become solid; and the lyo-lnminescence of 

 certain other materials when they are dissolved. Lastly, there is 

 the X-luminescence set up by the X-rays. 



Pausing on photo-luminescence, here is an experiment to illustrate 

 the difference between its two varieties, phosphorescence and fluor- 

 escence. Light from an arc lamp, filtered from all rays except 

 violet and ultra-violet, is thrown upon a disk to which rapid rotation 

 is given by an electric motor. The disk is painted with two rings, 

 one of sulphide of calcium, the other of tungstate of calcium. Though 

 the light falls only on one patch you note that the sulphide shows a 

 continuous ring of blue light, for the emission of light persists 

 after the stuff has passed out of the illuminating rays. The tungstate, 

 on the other hand, shows only a short trail of light, the rest of 

 the ring being non-luminous, since tungstate has but little persistence. 

 The light has in fact died out before the stuff has passed a quarter of 

 an inch from the illuminating beam. This is a sort of phosphoro- 

 scope designed to show how long different materials will emit light 

 after they have been shone upon. Those which show only a tem- 

 porary luminescence are called fluorescent, while those with persis- 

 tent luminescence are called phosphorescent. For many years it 

 has been known that some diamonds are phosphorescent. Three such 

 are here shown,* which, after exposure of one minute to the arc light, 

 shine in the dark like glow-worms. The most highly phosphorescent 

 material yet produced is an artificial preparation of sulphide of 

 calcium manufactured by Mr. Home. The specimen exhibited has a 

 candle-power of about yL candle per square inch after exposure for 

 a few seconds to direct sunlight; but the brilliancy rapidly dies 

 away, though there is a visible luminescence for many days. This 

 substance is also brightly luminescent in a Crookes tube, and less 

 brightly under the influence of X-rays. 



Many substances, notably fluor-spar, have the property of thermo- 

 luminescence, that is they shine in the dark when warmed. Powdered 

 fluor-spar dropj^ed upon a hot shovel emits bright light. If, however, 

 the spar is heated to a temperature considerably below red heat for 

 some hours, it apparently comes to an end of its store of luminous 

 energy and ceases to shine. Such a specimen, even after being kept 

 for some months, refuses to shine a second time when again heated. 

 It has, however, long been known that the property of luminescing 

 when warmed can be restored to the spar by passing a few electric 

 sparks over it, or by exposing it to the silent discharge or aigrette. 

 Wiedemann having found that the kathode rays produce a similar 

 effect, it occurred to me to try to find out whether any of these 

 X-rays also would revivify thermo-luminescence. I have found 

 that on exposure for twenty minutes to X-rays, a sample of fluor-spar 



* Kindly lent by Dr. J. H. Gladstone, F.R.S. 



