764 Prof. J. C. McLennan on the 



distributed over its surface. It was also very probably 

 distributed in a uniform manner throughout the mass 

 of the cylinder, as repeated scourings with glass-paper 

 failed to remove it. In this connexion it is of interest to 

 note that, during the last six months, measurements have 

 been repeatedly made on the conductivity of air confined in 

 this cylinder, but during that period no indication of a 

 falling off in the intensity of the radiation from it has been 

 observed. 



II. On the Ionization produced in Metallic Receivers by the 

 Secondary Rays excited by the Gamma Rays from Radium. 



From the foregoing results it is abundantly evident that 

 the high activity of lead, which has from time to time been 

 recorded by a number of observers, cannot be ascribed to 

 any intrinsic property of the metal, but must be connected 

 with the existence in it, in amounts varying with different 

 specimens, of some foreign body of considerable activity. 



It is known that part of the ionization in a gas confined in 

 metallic vessels must be due to the penetrating radiation 

 emitted by the earth, and part to the secondary rays excited 

 in the substance of the metallic receivers by these penetrating- 

 rays. From the results given above, part must also be due, 

 in some cases at least, to active impurities present in the 

 metal. 



The extremely low value found for the ionization with the 

 lead in cylinder No. 1, coupled with the value for " q " 

 obtained with zinc and aluminium receivers, suggests the 

 possibility that the materials out of which these vessels were 

 made were entirely or very largely free from active impurities, 

 and that the differences observed in the ionizations were due 

 to differences in the intensities of the secondary radiations 

 from the different metals. It is known that the secondary 

 radiation increases with the atomic weight of the metal 

 composing the radiating surface, and it seemed to the writer 

 possible that the difference in the values of " q y> found for 

 lead and aluminium, namely 23 and 15 ions per c.c. per 

 second, might be accounted for entirely on this ground. 

 • With the object of investigating this point an aluminium 

 cylinder was prepared from a thin sheet of the metal 0'41 mm. 

 in thickness, and a series of accurate measurements made on 

 the saturation current through the air which it contained. 

 A small quantity of radium bromide was enclosed in a block 

 of lead about 3 cms. in thickness, and placed at a distance 

 of about one metre from the aluminium cylinder. The 

 saturation current in the aluminium cylinder was again 



