Mr Cooke, Experiments on Penetrating Radiation. 163 



this cup was inverted and placed over the top of the vessel no 

 certain alteration in the ionization could be detected. 



Although the above experiments did not yield conclusive 

 evidence on the subject of the radiation from the atmosphere, they 

 showed that the radiation on the roof was not the same as that 

 inside the building, being at least 20°/o less than the radiation of 

 a similar character inside the laboratory. This increased radiation 

 inside the laboratory is not due to the fact that the building has 

 been used for experiments with radium or similar materials, for 

 the vessels when exposed in the rooms of a private house showed 

 if anything a trifle higher ionization, — certainly not less. The 

 explanation of the results up to this point is clearly that any 

 radiation which may be coming from the atmosphere is less 

 intense than that coming from the materials used in the construc- 

 tion of the building. 



The method of shielding the vessel from two directions on the 

 roof not yielding results with sufficient rapidity or certainty, the 

 next plan which suggested itself was to bury the apparatus, and 

 compare the leak under these conditions with that observed when 

 the vessels were exposed on the roof. In the one case the earth 

 surrounds the vessels on all sides ; in the other the atmosphere is 

 above the horizontal plane, solid material below. Comparing the 

 ionization under the two conditions should supply data for esti- 

 mating the ratio of the activity of the earth to that of the 

 atmosphere. Experiments carried out in this way show that the 

 difference in radiation under the two circumstances is very small, 

 if any exists. This however does not prove conclusively that the 

 radiation from the atmosphere is of the same order as that from 

 the earth, as there is always the possibility of the surface of out- 

 door objects giving furth a penetrating radiation to a greater 

 extent than the unexposed portions. The uncertainty in connec- 

 tion with this point has up to the present prevented definite con- 

 clusions from being drawn as to the activity of the atmosphere. 

 It is intended in continuing the work to test this point experi- 

 mentally if possible. 



Experiments were carried out with the apparatus submerged 

 3 feet below the surface of the Emmanuel College swimming 

 tank, which is about 7 feet deep. The ionization under these 

 conditions is about 12°/o l ess than when the vessels are exposed on 

 the roof. Comparing the results obtained indoors with those ob- 

 served with the apparatus submerged, they show a reduction similar 

 to that obtained by McLennan*, and the writer f, by shielding the 

 ionization vessels with water, working indoors. When the vessels 

 were allowed to float with their tops flush with the surface of the 



* Phil. Mag. v. p. 419 (1903). t Loc. cit. 



12—2 



