Phenomena by the Rontgen Rays. 233 



Having given a positive charge to the conductor (gas- 

 graphite, copper, zinc, &c.) on which the X-rays fall, I 

 observed the same action as in the case of the negative 

 charge. 



Accordingly, the IL-rays, unlike the ultra-violet ones, produce 

 the dispersion also of bodies electrified positively. 



Of course, while in the case of an initial positive charge the 

 dispersion ceases when there is on the conductor a charge equal 

 to that which, starting from the neutral state, the rays would 

 impart to it : on the other hand, with an initial negative 

 charge the dispersion continues until the body is discharged, 

 and then the same final result is obtained as it has when the 

 original charge is null or positive. This result may be ex- 

 pressed by saying that the conductor communicating with the 

 electrometer behaves like an electrode in a conducting medium 

 which acquires the potential of the region in which it is 

 placed, whatever was the initial potential. 



6. I desire to call attention to the fact that the new property 

 here mentioned has the advantage of furnishing a means of 

 measuring the X-rays. I believe that to compare the absorp- 

 tion produced by various bodies, it will be better to express 

 this absorption by the velocity of dispersion, rather than by 

 an estimation based on a comparison of the intensity of the 

 shadow projected on a fluorescent screen or on a photographic 

 plate. 



The new property will evidently have to be taken into 

 account when the time comes for discussing the various hypo- 

 theses proposed to explain the nature of the X-rays. 



7. The following is a lecture-experiment for showing the 

 dispersion of electricity produced by the Rontgen rays. The 

 metal box is dispensed with, retaining only the large lead 

 plate, with the window closed by aluminium. A disk of any 

 metal is placed at a few centimetres from the window on 

 one side of the lead plate, while on the other side is a 

 Crookes' tube with the apparatus for working it. The disk 

 is connected with a gold-leaf electroscope. The disk being 

 well insulated, a charge of either sign is given to the system 

 — the leaves remain divergent and motionless, but no sooner 

 is the Crookes' tube at work than they fall and become vertical. 

 If the aperture is closed with a large leaden plate, the phe- 

 nomena are no longer produced. If various other substances 

 are placed against the window, such as the hand, wood, &c, 

 the rate at which the leaves fall is slower. 



Phil. Mag. S. 5. Vol. 41. No. 250. March 1896. R 



