198 



Professor Silvanus P. Thompson 



[May 8, 



^ 



result of mere wave motions witliin the tube, tlie Britisli pliysiclsts, 

 including Lord Kelvin and Sir George Stokes, accept Crookes's view 

 of the material nature of the kathode rays. Who, indeed, that has 

 seen the molecule mill at work can doubt that, whether vibrations are 

 present or not (and doubtless there are vibrations present), there are 

 actually streams of moving particles as an essential feature of the 

 kathodic discharge ? For the moment the victory undoubtedly rests 

 with the views of Crookes. 



But of all these phenomena the one which concerns us most is 

 that of the production of electrical shadows. Erecting in the path 

 of the kathode rays an obstacle cutout in sheet metal — a cross of thin 

 aluminium is the favourite object— a shadow of it is observed to be 

 cast upon the wall of the tube behind it ; the glass phosphorescing 

 brilliantly except where shielded from the impact of the kathode rays, 

 so that the shadow comes out dark against a luminous background. 

 Common soda-glass gives this greenish-golden tint, while lead-glass 



exhibits a blue phosphorescence. Not 

 glass alone, but diamonds, rubies, emer- 

 alds, calc-spar and other earthy ma- 

 terials, such as alumina, and notably 

 yttria, produce the most brilliant effects 

 under the kathode discharge, some of 

 them only fluorescing transiently,other8 

 with a persistent phosphorescence. As 

 a sample is shown a tube in which 

 a sea shell, slightly calcined to remove 

 organic matter, is made to emit a bril- 

 liant luminescence under the impact of 

 rays from a kathode jilaced above it. 

 The shell itself casts a shadow against 

 the lower part of the tube. Some of the shadow effects are very 

 mysterious and have recently claimed much of my attention. The 

 size of the kathodic shadows is affected by the electrical state of 

 ,the object. Electrifying it positively makes its shadow shrink to 

 smaller dimensions. Electrifying it negatively causes a singular en- 

 largement of the shadow\ There seems to be no difference between 

 the shadow of a metallic body and that of a non-metallic body of the 

 same size. All bodies cast shadows, however thin. Even a film of 

 glass Toius i^c^ thick — so thin that it showed iridescence like a 

 soap bubble — was found by Crookes to cast its shadow. 



Another point noticed by Crookes was that if the exhaustion is 

 carried very far, and the tube is stimulated by a sufficiently strong 

 electromotive force, the phosphorescence may occur at points not in 

 the line of discharge but round a corner. Not that the kathode 

 rays turn the corner, however. Aj^parently some of the more quickly 

 moving or perhaps more highly charged particles — atoms, molecules 

 or ions — those, in fact, described by Crookes as " loose and erratic " 

 — would manage to get round the corners and produce effects of a 



Fig. 1. 



