566 Mr. A. Wood on Spontaneous Ionization of 



too gives as the result of similar experiments, that lead pro- 

 duces an ionization twice that for zinc, while tin is inter- 

 mediate between the two. This, again, is in agreement with 

 the table given above. If the ionization produced by the 

 various metals were really due solely to the presence of radio- 

 active impurities, it is inconceivable that vessels constructed 

 from different samples of the metals, and tested by entirely 

 different experimental methods, should give results in such 

 close agreement. It follows that the ionization due to a 

 metal has a certain definite value which is characteristic of 

 the metal — a conclusion which affords the strongest support 

 to the hypothesis that all matter is radioactive. 



The first screen used in these experiments was of lead, 

 about 1*3 cms. thick. The diminution of the ionization in a 

 lead vessel was measured in the way indicated above. With 

 a view to getting some idea of the degree of accuracy of the 

 results, three separate determinations were made. These 

 gave 9 per cent., 13 per cent., and 11^ per cent, respectively. 

 As this was the smallest diminution observed for any metal, 

 and as the probable error is greater the less the diminution, 

 it is improbable that the figures given in the following tables 

 as proportionate diminution are wrong by more than one, or, 

 at the very most, two units. Two determinations of the pro- 

 portionate reduction for the zinc vessel gave 26 per cent, 

 and 26^ per cent, respectively. A summary of the results for 

 each metal is appended. 



Lead. 



Unscreened. 



Screened. 



Unscreened. 



Screened. 



Unscreened. 



•540 



•507 

 •556 

 •534 • 



•490 

 •501 

 •485 

 •479 



•571 

 •545 



•528 

 •540 



•518 

 •485 

 •461 



•485 



•530 

 •540 

 •571 

 ■530 



Mean ...534 



•489 



•545 



•487 



•543 



Mean Ionization 



I Unscreened... *541 

 \ Screened '488 



Tin. 



Proportionate Diminution 10 per cent. 



Unscreened. 



Screened. 



Unscreened. 



•461 

 •496 

 •502 

 •513 



•493 



Screened. 



•400 

 •354 

 •393 

 •393 



•385 



Unscreened. 



•479 



467 

 •507 

 •496 



•487 



•496 

 •518 

 •496 

 •524 



Mean ...-508 



1 



•368 

 •387 

 •374 

 •406 



•384 



Mean Ionization {gJjJSSf 64 '" '|g4 Proportionate Diminution ^percent. 



