778 Radioactivity of Lead and other Metals. 



conductivity was under observation; and owing to the length 

 o£ time required for the maximum state to be reached, it is 

 not possible at present to express them as definite percentages 

 of the minimum reading obtained with this vessel. 



Repeated observations on both cylinders have invariably 

 given the results just described, and as the greatest care was 

 taken throughout the investigation to prevent contamination 

 of the cylinders by foreign active substances except what 

 might be introduced along with the filtered air, it would 

 seem clear that a process is going on in the metals, possibly 

 a diffusion from the interior, whereby the surface becomes 

 coated with a layer of active matter which makes an important 

 contribution to the ionizing power of the metal. 



From the observations which have been made so far, it has 

 been impossible to decide whether the second part of the rise 

 n the conductivity of the confined air was due, in whole or 

 in ] art, to an active substance introduced with the air or to 

 an emanation from the walls of the vessel, but as observations 

 are still being made with the cylinders, it is possible that some 

 additional facts may be obtained which will clear up this 

 difficulty, and also throw light on the nature of the active 

 impurity which has been shown to be present in varying 

 amounts in the different samples of lead examined. 



Y. Summary of Results. 



1. The conductivity of air enclosed in lead cylinders has 

 been shown to vary widely with the samples of lead selected. 

 The lowest conductivity observed in air enclosed by this 

 metal corresponded to the production in the air of 23 ions 

 per c.c. per second, and the highest to the production of 

 160 ions per c.c. per second. 



2. These wide variations show that the high activity of 

 lead which has been observed generally is due to the presence 

 of active impurities in varying amounts in the lead, and not 

 to a high intrinsic radiation from the metal. 



3. Calculations made on the observations show that the 

 differences in the conductivities of air confined in vessels of 

 different metals, including lead, when free from active im- 

 purities, arise from and are due to differences in the secondary 

 radiations from these metals. 



4. Experiments made with the gamma rays from radium 

 showed that of the ionization produced by these rays in air 

 enclosed in lead receivers, two-thirds was due to the excited 

 secondary rays and one-third to the gamma radiation itself. 



