164 Transactions of the Canadian Institute. [vol. ix 



air was taken from the apparatus. It will be seen also on comparing the 

 curves that the initial point of the upper curve is much higher than the 

 initial points of the lower curves. Also the upper curve decreases much 

 more quickly than the lower curves, until finally the three curves all 

 come together. Now the decrease in the ionisation current through the 

 air space with the time would be the same in all three experiments be- 

 cause this decrease is dependent on the air pressure and, as the pump was 

 kept going regularly the air pressure in all three cases would be the same 

 at equal intervals of time after the pump was started. Hence since the 

 upper curve shews a much greater drop than the two lower curves, there 

 must have been a much greater decrease in the secondary radiation in 

 the first experiment than in the two latter experiments. Therefore there 

 must have been a much more copious emission of secondary rays at the 

 beginning of the first experiment than at the beginning of the other two 

 experiments. This is indicated by the height of the initial point of the 

 upper curve above the initial points of the other two curves. 



The above experiments therefore shew that when fresh carbon is 

 used as an electrode and subjected to bombardment by a rays and the 

 air withdrawn from around the carbon, there is a great decrease in the 

 secondary rays as time goes on until finally the secondary rays emitted 

 reach a constant value. Then, if the carbon is kept in vacuo for some 

 time (one week) it will not regain its primary power of emitting secondary 

 rays by being again surrounded by air while still under bombardment 

 by a rays even for periods of time extending up to 22 hours. 



The next experiments that were performed were for the purpose of 

 finding out whether this same carbon would send out secondary rays 

 with their initial intensity if it were placed in an atmosphere of hydrogen 

 instead of air. The apparatus was first filled with dry hydrogen, pro- 

 duced from zinc and acidulated water and left at atmospheric pressure 

 for four hours. Then as before a positive charge of 80 volts was put on 

 the polonium, the pump was started, and at a definite interval of time 

 after starting the pump, a series of readings was taken of the rate at 

 which the carbon electrode charged up, and the pressure of the hydrogen 

 in the apparatus. The readings are given in the following table. 



