296 Dr. Norman Campbell on the 



hydrogen carrying 2 milliamperes for two hours; the only 

 effect was to produce the temporary decrease of Z already 

 mentioned. At length, however, by allowing 200 volt rays 

 to fall on P from W (current about 10 " 9 amp.) for 24 hours 

 in a pressure of about 0*01 of a mixture of hydrogen and 

 tap-grease Z was increased again to 1*2, very nearly its 

 original value. The orio-mal state of the metal had not been 

 reproduced however; for while formerly three days of heating- 

 had been necessary to reduce Z to 0*3, now Z fell to 0*23 

 after two minutes' heating, during which a large quantity of 

 gas was evolved. 



11. In the foregoing account the history of only one piece 

 of platinum has been traced. Less complete observations 

 have been taken in slightly different forms of apparatus on 

 other specimens. All agreed very closely in giving curve A 

 initially, and all could be made to give curve B after a few 

 days of heating. An attempt was made to produce state B 

 from state A initially by spluttering the surface of A in a 

 discharge; in two hours the discharge produced no effect, 

 -though the walls of the tube were completely blackened ; a 

 further continuance for \ hour brought the experiment 

 to an end by the covering of the insulation with the 

 deposit. 



12. The facts which have been recorded may be briefly 

 summarized : — The ionization produced by cathode rays in 

 platinum can be reduced very greatly by continued heating 

 of the platinum in a vacuum and in oxygen. At first the 

 heating produces no effect, then it produces a large and 

 sudden change from state A to state B, followed by a very 

 slow and long-continued change the end of which has not 

 been reached. It is quite possible, or even probable, that 

 ultimately a state would be reached in which the platinum 

 would not be ionized at all by rays of less than 40 volts speed. 

 The initial state A of the metal can be restored by allowing 

 cathode rays to fall on the metal with a speed of some 

 100 volts in a pressure of gas less than 0'01 mm. ; mere 

 access to the metal of gases and vapours does not restore 

 that condition quickly, if at all. And when the initial con- 

 dition is restored by the action of cathode rays, it is much 

 -less permanent than before, and state B can be regained by 

 a few minutes, instead of many hours, of heating. There is 

 no evidence that the passage of a luminous discharge produces 

 any permanent effect, whether the platinum is anode or 

 cathode ; but in either case it produces a decrease in the 

 ionization, which is reversed immediately cathode rays fall 

 on the metal. 



