150 Mr. William Ctvol-es [April 4, 



plios])liorcsceiico. Sometliiiiij: is projoototl from the negative polo 

 ^vlnl'll has tlie power of hainiuering luvay i^t the ghiss in front of it, in 

 sueh a way as to eause it not only to vibrate and beeonie temporarily 

 luminous wliile the diseluxrge is going on, hut to produee an im- 

 pression njHni the glass wliieli is permanent. The explanation whieh 

 has gradually evolved itself from this series of experiments is this : — 

 The exluxustiou in tliese tubes is so high tliat the dark spaee, as I 

 sliowed you at the commencement of this Lecture, that extended 

 around the negative pole, has widened out till it entirely tills the 

 tube. By great rarefaction the mean free patli has become so long 

 tliat the hits in a given time may be disregarded in comparison to the 

 misses, and the average molecule is now allowed to obey its own 

 motions or laws without interference. The mean free path is in fact 

 comparable to tlio dimensions of the vessel, and we liave no longer to 

 deal with a coufinitoiis portion of matter, as we sluinld were the tubes 

 less highly exhausted, but we must here contemphite the molecules 

 iiidiriditalh/. At th'st this was only a convenient working hypothesis. 

 Long-eontinned experiment then raised this provisional hypothesis 

 almost to the dignity of a thet)ry, and now the general oj)inion is that 

 this theory gives a fairly correct explanation of the facts. In these 

 highly exhausted vessels the mean free path of the residual molecules 

 of gas is so long that they are able to drive across from the pole 

 to the other side of the tube with comparatively few collisions. The 

 negatively clectritied molecules of the gaseous residne in the tube 

 therefore dash against anything that is in front, and cast shadows 

 of obstacles just as if they were rays of light. Where they strike the 

 ghxss they are stopped, and the production of light accompanies this 

 sudden arrest of velocity. 



Other snbstanees besides English, German, and nranium glass, and 

 Beequerel's luminous sulphides, are also phosphorescent. I think, 

 without exception, the diamond is the most sensitive substance I have 

 yet met for ready and brilliant phosphorescence. I have here a tube, 

 similar to those already exhibited, containing a mica screen painted 

 with powdered dianunid, and when I turn on the coil, the brilliant 

 blue phosphorescence of the diamond can be seen, quite overpowering 

 the green phosphorescence of the glass. Here, again, is a very 

 curious diamond, which I was fortunate enough to meet with a short 

 time ago. By daylight it is green, produced, I fancy, by an internal 

 fluorescence. The diamond is mounted in the centre of this ex- 

 hausted bulb (Fig. 10), and the negative discharge will be directed 

 on it from below npwards. On darkening the theatre you see the 

 diamond shines with as much light as a candle, phosphorescing of a 

 bright green. 



In this other bulb is a remarkable collection of crystals of 

 diamonds, wliich have been lent me by Professor Maskelyne. When 

 I pass the discharge over them I am afraid you will only be able 

 to see a few points of light, but if yon will examine them after 

 the Lecture, you will see them phosphoresce with a most brilliant 



