of Ordinary Materials. 683 



It was found that ignition did not affect the radioactivity of* 

 .a given sample of platinum. 



In order to compare the activity of the substances men- 

 tioned above with that "of uranium salts, a small crystal of 

 uranium nitrate measuring 12 x 4 mm. was cemented to the 

 inside of the cylinder. The rate of leak due to it was found 

 to be thirteen times that due to the most active specimen of 

 platinum. The area of the uranium nitrate was only ^hw 

 part of that of the platinum, so that its activity for an equal 

 area would be no less than 3000 times greater- 



It is possible that the radioactivity of ordinary materials 

 may be due to traces of the more active substances. This 

 would explain the varying activities of different samples of 

 the same material. Only an infinitesimal proportion of ra- j 

 dram would be required. Radium is at least 100,000 times ; 

 more active than uranium, and uranium 3000 times more 

 active than the most active common material that I have 

 experimented with. So that one part of radium in three 

 hundred million would suffice to account for the observed 

 effects. ...... „ i 



It was evidently important to compare the quality of the 

 Becquerel rays emitted by the various substances. This can 

 be done, in the case of powerful radiators like uranium or 

 radium, either by examining the deflexion in a magnetic 

 field, or by observing the amount of absorption by solid or 

 gaseous, media. In the case of very feeble radiators it would 

 be quite impracticable to attempt the magnetic experiment, 

 while the absorption by solids can scarcely be examined, 

 since the solid absorbent would give off radiation of its own 

 comparable in amount with that due to the original radiating 

 substance. We are reduced, then, to examining the absorb- 

 tion by air'"*. 



In order to compare various radiations in this way we have 

 only to examine the variation of leak with pressure. In the 

 case of the more absorbable radiations less pressure will be 

 required to make the current sensibly independent of a 

 further increase of pressure, or, what is the same thing, to 

 absorb practically all the radiation. 



In the experiments the apparatus used was similar to that 



* It may be remarked that in all probability some part of the observed 

 ionization of air may be due to radioactivity of the surrounding- air in 

 the other parts of the vessel. But it is not apparent how this can bo 

 experimentally distinguished from the ionization due to the walls. In 

 all probability this ionization due to surrounding- air is small, since it is 

 scarcely possible to understand otherwise how the ionization for largo 

 pressures could be sensibly independent of pressure, 



