189 6. J on Electric Shadoivs and Luminescence. 207 



the X-rays than tliat of employing tlie liiglily exhausted vacuum tube. 

 They are not found in the light of ordinary electric sparks in air. 

 They are not discoverable amongst the rays emitted by ordinary 

 Geissler tubes with a low exhaustion. They are not found in sun- 

 light or any artificial light. The arc light, though it yields rays 

 that will give photographic shadows through a thin pine-wood 

 board, yields no rays, that will pass through aluminium. The only 

 other rays that seem to come within reasonable possibility of being 

 X-rays are the Lenard rays, some of which are probably identical 

 with Roentgen's ; the Wiedemann rays, which are, so far as yet investi- 

 gated, entirely similar ; and the Becquerel rays, to which some allusion 

 will presently be made. It will, however, be convenient here to 

 present a synoptic table (see p. 208) of the various kinds of rays and 

 their respective physical properties. 



One other physical property of the X-rays has been discovered 

 since the publication of Eoentgen's research. It was discovered 

 simultaneously in Cambridge (by Professor J. J. Thomson), in Paris, 



in Bologna, and in St. Petersburg, that these X-rays 



will cause the diselectrification of an electrified body, no f" ''\ 



matter whether it is positively or negatively charged.* / Q \ 

 That ultra-violet light can diselectrify bodies that have / ^^^ \ 

 been negatively charged was previously known from the J^^^^^^ 

 researches of Hertz, and of Elster and Geitel. This 

 fresh discovery that X-rays will also discharge a posi- 

 tive electrification sets up a new physical test. Let me 

 show you a simple piece of apparatus which I have found 

 very convenient for the purpose of demonstrating this 

 discovery. It is an aluminium-leaf electroscope (Fig. 9) 

 entirely shielded from all external electrostatic influences 

 by being enclosed in transparent metallic gauze. It Fig. 9. 

 is so well shielded that even when the cap is removed 

 it cannot be charged in the ordinary inductive way, but must be 

 electrified by direct conduction. The aluminium leaves hang at the 

 side of a fixed central plate as in Exner's electroscope. The con- 

 taining vessel is of thin Bohemian glass. On exciting the instru- 

 ment positively from a rod of rubbed glass, or negatively from a 

 rod of rubbed celluloid, the leaves diverge. In either case as soon 

 as the X-rays are caused to shine upon the instrument the leaves fall. 



It occurred to me that by the aid of this property of diselectrifica- 

 tion it might be possible to produce electric shadows without having 

 resort to any photography. You are aware that if the surface or 

 any part of the surface of a body is electrified, the fact that it 



* It is of great interest to note that this identical property had been observed 

 by Lenard a year previously as an effect of his rays. He found they would dis- 

 charge an electroscope enclosed in a metal chamber, with an aluminium sheet in 

 front, whether positively or negatively charged, and at a distance of 30 centi- 

 metres from his tube. 



