A Gas generated from Aluminium Electrodes. 779 



With aluminium cylinders on the other hand, the measure- 

 ments show that approximately two-thirds of the ionization 

 was due to the gamma rays, and one-third to the secondary 

 rays excited by this radiation in the metal. 



5. Calculations based on observations on the conductivity 

 of air confined in different receivers lead to the conclusion 

 that approximately 9 ions per c.c. per second are generated 

 in free air by the penetrating radiation from the earth. 



Before concluding I wish to acknowledge my very great 

 indebtedness to Mr. V. E. Pound, for his kindness in repeating 

 and verifying many of the observations described in the first 

 part of this paper. 



The Physical Laboratory, 

 University of Toronto, 

 July 1, 1907. 



LXXVI. A Gas generated from Aluminium Electrodes. 

 By R. v. Hirsch, Ph.D., and F. Soddy, M. A* 



IN a recent paper (R. v. Hirsch, Phys. Zeit. vol. viii. p. 461, 

 1907) one of us showed that when cathode rays were 

 generated by means of an influence machine in pure gases, 

 the relation between the gas pressure p and the discharge 

 potential V could be represented by 



p 2 V = constant. 



This relation holds for the pure gases examined independently 

 of the amount of current flowing through the tube, but does 

 not hold at all for gaseous mixtures. During the passage of 

 the discharge, a gas is continuously evolved from the electrodes 

 if these are of aluminium, which renders the gas initially 

 filling the tube impure, so that the above relation ceases 

 to hold. But if the discharge is passed for some hours, 

 the gas being pumped out as evolved so as to maintain the 

 pressure within the range required for the production of 

 cathode rays without unduly r increasing the resistance of the 

 tube, the value of p 2 Y again becomes a constant independent 

 of the nature of the gas initially filling the tube and about 

 one-quarter of the value for hydrogen. 



The value of p 2 Y appears to depend in some way on the 

 molecular weight of the gas, for it is almost exactly fourteen 



* Communicated by the Authors. 



