298 Dr. Norman Campbell on the 



some gas) I' returned to its original value and Z' to near its- 

 original value. Usually increases but sometimes decreases 

 in T /J were observed. There appeared to be no relation between 

 the changes and the temperature of P ; no changes were 

 observed unless P was heated till it just emitted light, but 

 they did not increase consistently when the temperature was 

 increased to a full yellow heat. The inconsistency of the 

 results is disturbing, but the observations are not easy ; the 

 sagging of the foil when heated doubtless introduced com- 

 plications. The matter is not complely elucidated, but I 

 think it is clear that there is no direct effect of temperature 

 on ionization. 



14. In all the experiments which have been described so- 

 far the speed of the cathode rays incident on the platinum 

 was less than 40 volts. It is obviously interesting to inquire 

 whether the difference between states A and B persists when 

 rays of much greater speed are used ; for example, would 

 the difference persist if the cathode rays corresponding to 

 hard X rays were used? If it does not persist for rays of 

 all speeds, then at some point the B curve must rise more 

 steeply than the A curve; is this rise initiated by a 'kink/ 

 similar to that which occurs at 11 volts, indicating the 

 attainment of the ionization potential for the metal itself ? 



These questions have not yet been investigated thoroughly, 

 but there is no indication of an approach of curve B to 

 curve A when the speed of the incident cathode rays is 

 increased to 100 volts, and below this speed there is no 

 evidence whatever of a second ' kink' which should iudicate- 

 ionization of the metal. 



15. Some observations were made of the speed of the rays* 

 electrons leaving P in an apparatus without the tube T ; the 

 information can be obtained by varying V 3 . It was pointed 

 out in § 1 that the passage of the incident rays through the 

 speed "W is marked, not only by an increase in P, but also 

 by a change in the distribution of the velocity of the electrons 

 " reflected." It was examined whether this change took 

 place when the platinum was in state B as well as when it 

 was in state A. The conclusion was affirmative ; the state 

 of the platinum does not seem to affect the velocity of the 

 electrons leaving P. Indeed the change in the speed curve 

 of those electrons is often a more delicate test of the position 

 of the ionization potential than direct measurements of P. 



16. Now let us inquire what conclusions may be drawn 

 from the facts which have been described. An explanation) 

 of them, on the whole satisfactory, can be offered in terms of 



