1896.] on Electric Shadows and Luminescence. 197 



discbarge, which only occurs at a very high degree of exhaustion, 

 possesses several properties which distinguish it from all other kinds 

 of discharge. It is propagated in straight lines, causes a brilliant 

 luminescence wherever it strikes against the glass walls of the 

 tubes, casting shadows of intervening objects, it heats the surface 

 on which it impinges, and strikes them with a distinct mechanical 

 force. Singular to relate, it is also capable of being deflected 

 by a magnet as though it were a flexible conductor carrying 

 the current. Struck by the singularity of these kathode rays or 

 kathode discharges, which formed the subject of several beautiful 

 researches, Crookes advanced the hypothesis that they consisted of 

 flights of negatively electrified particles or "radiant matter." The 

 particles he sometimes spoke of as molecules, sometimes as dis- 

 sociated atoms, or, as we should now say, ions. He studied the wan- 

 derings of these flying particles by inserting within the bulb at 

 different points auxiliary electrodes. He found the interior of the 

 bulb to be positively electrified in all parts except within the dark 

 space which surrounds the kathode, that is to say, except within the 

 range of the actual kathode discharge. The kathode discharge itself 

 was found to be possessed, to an extent exceeding any other known 

 agency, of the power of exciting fluorescence and phosphorescence in 

 minerals and gems. The kathode rays were themselves discernible as 

 they crossed the interior of the tube. In such a bulb the kathode rays 

 would form a blue streak impinging straight upon the anode. The. 

 kathode used in the next Crookes tube, is of a concave shape. 

 Crookes found that, since the kathode rays left the surface normally, 

 the result of curving the kathode was to focus the rays toward the 

 centre of curvature. By so focussing the rays upon a bit of platinum 

 foil, it was found possible to fuse and even melt the metal. 



Unlike the discharges obtained at lower stages of rarefaction, the 

 direction of these kathode rays was found to be independent of the 

 position of the anode. He found kathode rays to be produced even 

 when no internal electrodes were inserted, and when, instead, external 

 patches of tinfoil were attached to the glass. Their mechanical action 

 he studied by causing them to impinge upon the vanes of a pivoted 

 fly which w^as thereby set into rotation. In a later experiment he 

 caused the fly of a " molecule mill " to be set into rotation, not by the 

 impact of the kathodic discharge but by the kinetic energy of tlie 

 particles returning back toward the anode after they had impinged 

 against the walls of the tube and lost their negative electric charges. 

 A mere resume of Crookes's work in those years beginning about 

 1869 or 1870, and extending not only for ten years^ctively, but going 

 on at intervals until a year or two ago, would of itself fill a whole 

 course of lectures. Into the controversy which has arisen between 

 Crookes and the English physicists on the one hand and the German 

 physicists on the other, there is no need to enter. Suffice it to say 

 that while the German physicists mostly reject Crookes's hypothesis 

 of radiant matter, and regard all these various phenomena as the 



