570 Mr. A. Wood on Spontaneous Ionization of 



is mainly secondary the proportionate diminution due to a 

 screen will be greatest, while in those cases where an 

 intrinsic radiation from the walls predominates the propor- 

 tionate diminution due to a screen will be least. Applying 

 this criterion to the metals examined, we find that in tin, 

 iron, and zinc the radiation must be largely secondary, 

 while in lead and aluminium it must be largely intrinsic. 

 The results for the iron screen are briefly summarized in the 

 following table : — 



tvt ^ ' 1 Ionization Ionization Diminution Diminution 



a ena . ( unscre ened). (screened). (absolute), (proportionate). 



Lead -524 -472 -052 10 p. cent. 



Tin -504 -435 '069 12J „ 



Iron -298 '226 '012 24 „ 



Aluminium... -287 -243 '044 15 „ 



It will be seen that the proportionate diminution due to 

 this screen is in three out of the four cases identical with 

 that produced by the lead screen. The exception is tin, where 

 the difference is very marked indeed. Nor is this discrepancy 

 due to experimental error, as the determination was carefully 

 repeated and confirmed. 



Having thus established a probability in favour of the view 

 that all matter is radioactive, a systematic search for emana- 

 tions from the various metals used was commenced. The 

 method adopted was to heat the vessel and endeavour by this 

 means to drive out any emanation which was stored up in 

 the metal. Preliminary experiments ^ave every indication 

 of the existence of such emanations. When the vessel was 

 heated the leak increased very greatly at first, but soon fell 

 off again, and when the vessel was allowed to cool, the leak 

 fell to a very low value, which seemed on the face of it to 

 correspond to the leak given by a de-emanated metal. The 

 apparatus employed was exactly the same as that previously 

 described, except that care was taken to screen the electroscope 

 from the source of heat, which in this case was a Bunsen 

 burner. Lead, brass, and tin were used, and all three gave a 

 result similar in kind but differing in degree and in the time 

 constant. The apparatus was now duplicated, and two vessels 

 were used, each with an electroscope. The vessels were fitted 

 with two delivery-tubes ; in the first vessel one of those tubes 

 was connected to a long indiarubber tube which passed out 

 into the open air, while the other communicated with one of 

 the delivery-tubes of the second vessel ; the second tube from 

 this vessel was connected to a pump. A current of air could 



