770 Prof. J. 0. McLennan on the 



was almost exactly the inverse of the preceding, the ioniza- 

 tion due to the primary rays being nearly twice that due to 

 the corresponding secondary radiation. 



It will be remembered, too, that the radium from which 

 the gamma rays were obtained w T as surrounded by a block 

 of lead 3 cm. in thickness, so that the radiation which issued 

 from it must have been of a very penetrating nature, and 

 therefore similar in its characteristics to the penetrating 

 radiation which has its source in the earth, and contributes 

 to the natural ionization observed in air or other gases con- 

 fined in metallic vessels. 



It seems fair to conclude then, that in natural or spon- 

 taneous ionization in air confined in metallic vessels a pro- 

 portion should hold between the ionization due to the primary 

 and that due to the secondary rays, similar to the one which 

 was found to hold experimentally with the gamma rays from 

 radium, and the secondary rays emitted by them. 



Assuming this relation to hold, it is possible to establish 

 a connexion between the conductivity of air confined in a 

 vessel of one metal with that of air enclosed by a second of 

 the same dimensions but of different material, provided 

 neither metal contains any radioactive impurities. 



With this relation established it is possible then to check 

 the results obtained experimentally in particular cases, and 

 by so doing arrive in a measure at a knowledge of the re- 

 lative importance of the different factors which determine 

 the ionization. 



In Section L of this paper it has been shown that with the 

 lead cylinder No. 1 there was generated on the average 

 23 ions per c.c. per second. Assuming that no part of this 

 was due to any impurity in the metal, it follows from the 

 numbers given in Table V. that one-third of this number 

 was due to the penetrating radiation which entered tho 

 cylinder, that is 7*67 of the 23 ions were generated by the 

 penetrating radiation which traversed the air in the vessel. 

 Allowing for the absorption by the cylinder of 15\36 per cent, 

 of the penetrating radiation, it follows that 9'06 ions were 

 generated per c.c. per second in free air by the penetrating- 

 radiation from the earth. Turning now to the aluminium 

 cylinder No. 10, it is fair to assume, since its absorption of 

 the gamma rays has been shown to be negligible, that 9*06 

 may be taken, without sensible error, to be the number of 

 ions generated per c.c. per second by the penetrating radiation 

 which entered it. The number produced per c.c. per second 

 by the induced secondary radiation would then, according to 

 Table Y., be 57 per cent, of this number, that is 5*1 6, and 



