430 Sir J. J. Thomson 



We can test this point in the following way : — A perfo- 

 •ated cathode C (fig. 3) is fixed in the middle or a sym- 

 netrical tube, and anodes A and B are placed symmetrically 



Cai 

 A 



Fig. 3. 

 C 





Anode \ jf^M m^%~ J Anode 



on opposite sides of it. If one anode, say B, is insulated the 

 Oanalstrahlen can be seen travelling down the B side of the 

 tube, while if A is insulated and B used as an anode they 

 travel down the A side of the tube. If, now, A and B are 

 connected together and used simultaneously as anodes, dis- 

 charges take place on both sides of the cathode, the Crookes 

 dark space is developed on both sides, and extending through 

 and far beyond these are the two pencils of Oanalstrahlen. 

 These are now somewhat thicker close to the cathode than 

 they were when the discharge took place in only one- half of 

 the tube, as if some had been stopped close to the cathode ; 

 but these so stopped form only a small fraction of the whole, 

 the great majority travelling on right through the dark space 

 on the side of the cathode opposite to that in which they 

 were generated. This may be due in part to many of the 

 Oanalstrahlen being uncharged for a time after they have 

 passed through the cathode ; but if this were the complete 

 explanation, the fall of potential in the dark space must be 

 concentrated quite close to the cathode, so that the velocity 

 of the Oanalstrahlen, if this were due to the attraction by the 

 cathode of the positive particles, would be very small until 

 the particles got quite close to the cathode. 



Now it can be shown that the Oanalstrahlen possess their 

 characteristic properties long before they reach the cathode. 

 One way of doing this is as follows : — One of the most charac- 

 teristic properties of the Oanalstrahlen is their power of 

 exciting the sodium radiation in suitable sodium salts. I 

 find that one of the best ways of getting a surface sensitive 

 to these rays is to use a flat perforated cathode and plug up 

 the whole with a short capillary tube made of soda-glass. 

 When a vigorous discharge is sent from this cathode the tip 

 of the tube shines with a bright yellow light, and some salt 

 of sodium splinters off from the glass and is deposited on the 

 adjacent parts of the cathode. This deposit is exceedingly 

 sensitive to Oanalstrahlen, and the places where these strike 



