772 Prof. J. C. McLennan on the 



only from the surface of the different radiators, but from a 

 considerable depth as well; and since the zinc used in these 

 experiments was only 1'62 mm. in thickness, it was probably 

 not so thick as the plates used by him. A smaller value 

 should then be assigned to the ionization produced by the 

 secondary radiation from the zinc walls ; and if this were 

 done, the calculated and the observed values for zinc would 

 come into better agreement. 



The argument which has just been used to explain the high 

 ionization calculated for the zinc cylinder would apply with 

 still greater force to the secondary rays from aluminium. 

 With this metal Eve found that the secondary radiation came 

 from as great a depth as 3 mm.; and if this condition holds 

 generally for aluminium, it follows that we have assigned for 

 this metal also too high a value to the ionization produced 

 by the secondary rays. A reduction should then be made in 

 the calculated value for " q " of 14"22 ions per c.c: per second, 

 and as this value is already slightly below the observed value 

 of the number of ions generated per c.c. per second in the 

 aluminium cylinder, this reduction would leave a correspond- 

 ingly greater number of ions to be accounted for, very 

 probably by the presence of active impurities in the substance 

 of the receiver. 



It is of special importance, however, to note the fair agree- 

 ment which exists between the calculated and the observed 

 values, in these experiments, for the ionization produced in 

 air enclosed in cylinders of lead, zinc, and aluminium, as 

 illustrated by the numbers given in Table VI. for Cylinders 

 Nos. 1, 8, and 9, since it emphasizes the view that ordinary 

 metals do not possess any intrinsic radiation, and that when 

 any high conductivity is observed in air confined in metallic 

 vessels, it must be due to the existence of quantities, more or 

 less considerable, of some foreign radioactive substance in 

 the metals. 



Examples of such contamination are clearly in evidence in 

 the results given in Table VI. for the lead cylinders Nos. 2 to 7 

 inclusive; and the numbers given in Column 6 give an 

 estimate of the relative amounts of the active impurities 

 present in the different samples of lead used in their con- 

 struction. 



In what has preceded in this Section the discussion has 

 rested upon the assumption that the lead in Cylinder No. 1 

 contained no active impurity; and while the experimental 

 results rather fit in with the deductions which have been 

 made on this hypothesis, there still remains the possibility 

 that some part of the ionization observed with this cylinder 



