Radioactivity of Lead and other Metals. 761 



exhibit an activity less intense than that which it emitted 

 when freshly mined. 



Eve does not appear to have tested many different samples 

 of lead, but if the explanation offered by Elster and Greitel 

 of the high activity of lead be correct, one should expect 

 to find that samples of lead selected at random from 

 different localities would exhibit widely differing degrees of 

 activity. 



Such a difference in the radioactivity of lead obtained from 

 different sources was recently observed by the writer while 

 making some measurements on the conductivity of air 

 contained in metal vessels. 



In these experiments the metals examined were made up 

 into cylinders 60 cm. long and 24 cm. in diameter, and from 

 measurements with a sensitive quadrant electrometer on the 

 saturation current through the air which they contained, their 

 activities were deduced. 



The experiments were conducted in a room free from any 

 artificial contamination ; and in carrying them out, the 

 cylinders were first carefully cleaned with glass paper and 

 then thoroughly washed out with hydrochloric acid, water, 

 ammonia, and ethyl alcohol, and finally, before making the 

 measurements, air filtered through glass and cotton-wool 

 was blown through each of them for fifteen or twenty minutes. 

 The results obtained with the different metals examined are 

 contained in Table I. (p. 762). 



From this table it will be seen that the values of " q " for 

 aluminium and zinc are somewhat less than those found by 

 Eve for this constant with the same metals. They are, how- 

 ever, in good agreement with H. L. Cooke's corrected value 

 " q" = lo m 6 given by Eve for air confined in a well-cleaned 

 brass vessel. 



The values found for "#" in the experiments with lead 

 cylinders, as will be seen from the table, range from 23 to 

 160 ions per c.c. per second. The lowest value, 23, was 

 obtained with the lead which had been in the laboratory 

 between twenty-five and thirty years, and had probably been 

 a very much longer time away from the mine. "With the 

 cylinder No. 4, which was made from an old drain-pipe, the 

 value of " q " was found to be 78, a somewhat higher value 

 than than obtained with No. 1. Although both of these 

 cylinders were made of comparatively old lead, it is highly 

 probable that No. 4, from the nature of its use, had become 

 contaminated with some active substance. It may possibly 

 too have possessed a higher activity than No. 1 when 

 originally mined. 



