ELECTRIC DISCHARGE IN GASES 117 



capable of passing through it, at any rate it appeared to do so, as 

 phosphorescence was excited on a glass surface a short distance 

 from the opposite side. Such a fact was, however, difficult to 

 reconcile with Crookes' hypothesis of electrified particles of the 

 same dimensions as the molecules of ordinary matter. In 1895 

 the X-rays were discovered by Rontgen, and these X-rays, 

 which are not material, are produced by the cathode rays when- 

 ever they strike matter of any kind. The characteristic property 

 of the X-rays is their power to penetrate more or less easily a 

 great variety of substances opaque to ordinary light. It was 

 observed by Rontgen almost immediately that when the hand 

 is held in the path of these rays cast on a fluorescent screen, the 

 shadow of the bones is darker than that of the flesh. Similarly 

 these rays will pass through paper, wood, and metals of small 

 atomic weight, such as aluminium, with little diminution, though 

 they are completely stopped by a comparatively thin layer 

 of a metal of high atomic weight, such as platinum, gold, or 

 lead. 



As the particles shot off from the cathode are material and 

 carry an electric charge, the path they follow may be regarded 

 as the path of an electric current, and accordingly they are 

 deflected by the action of a mag- 

 netic or electric field. Measure- 

 ments and observations of the 

 effects produced led Sir J. J. 

 Thomson to conclusions of the 

 greatest importance as to the Fla 49 ' J- J- THOMSON'S APPARATUS. 

 velocity and mas? of these particles. The apparatus used is 

 shown diagrammatically in Fig. 49. Here C is the cathode, 

 A is the anode, and B B' are two metal plates with a hole 

 through the centre by which a pencil of cathode rays is con- 

 ducted down the axis of the tube between the two parallel 

 plates of aluminium, D D', which can be charged as required. 

 A and B B' are connected to earth. When the discharge is 

 passing the pencil of cathode rays falls on a zinc sulphide screen 

 producing a spot of bright green light at F. If a difference of 

 potential is established between the two plates D and D' the rays 

 are bent down to some point F', and the radius of the circle into 

 which they have been deflected can be calculated. The ratio of 

 the mass of the cathode particle to the charge it carries can be 

 calculated from the data thus obtained. 



