560 Mr. A. Wood on Spontaneous Ionization of 



A second iron screen, 5 cm. thick, gave the following 

 readings : — 





Initial 

 Reading. 



Final 

 Reading. 



Zero 

 Change. 



Deflexion. 



Without 

 Screen. 



With Iron 

 Screen. 



17-4 

 210 

 221 



18-8 



22-3 

 11-9 



79-3 

 63-8 

 721 

 G5-3 



64-7 



48-8 



29 

 8-6 

 0-3 

 1-9 



Mean ... 



1-4 

 2-4 



Mean ... 



50-0 

 50-6 

 50-3 

 44-6 



48-9 



43-8 

 345 



39-1 



Diminution due to Screen, 20*0 per cent. 



Thus the second screen, although about five times the 

 thickness of the first, produced a diminution only a little more 

 than twice as great. The paper screen w T as now retried — the 

 apparatus being now surrounded with books to an average 

 thickness of about 35 cm. The effect on the ionization was 

 extremely small, but a diminution amounting to about 3 per 

 cent, could be distinctly observed in the mean of a number 

 of experiments. Thus the experiments show that, with the 

 exception of wood, all the materials tried as screens reduce 

 the ionization, and there seems therefore to be every reason 

 for concluding that the ionization is due, in part at least, to a 

 radiation of a very penetrating kind coming from some 

 external source. The result obtained in the case of wood 

 may be very simply explained on the hypothesis that the 

 increase of ionization is due to a radiation of very feeble 

 penetrating power emitted by it, which more than compen- 

 sates for the absorption produced by a screen of the thickness 

 used. This hypothesis is further strengthened by the fact 

 that the effect is entirely cut off by a thin screen of lead, and 

 also by the fact that Cooke * found an exactly similar result 

 with' bricks. That every material does absorb the penetrating 

 radiation to some extent, is rendered probable by the fact 

 that even paper when in sufficient thickness was found to 

 effect a small reduction of the ionization. It has been sug- 

 gested by J. J. Thomson that every material both emits and 

 absorbs a penetrating radiation capable of ionizing a gas, and 

 that the effect produced on the ionization in a vessel by a 

 screen of any material will depend on the ratio between the 



* Phil. Mao-. October 1903. 



