the Radiations from Radioactive Substances. 17 



With the sample of radium obtained from P. de Haen, Han- 

 nover, the amount of excited radiation obtained in this way 

 was small. In order to obtain intense excited radiation the 

 following course was adopted. 



A small amount of the radium compound was placed on a 

 small platinum plate which rested on a larger iron plate. A 

 sheet of asbestos with a rectangular hole in the centre, smaller 

 in area than the platinum plate, was placed over it. The 

 platinum plate to be radioactive was placed over the hole in 

 the asbestos and an iron plate placed on top of it. The top 

 platinum plate was connected to the negative pole of a 

 battery of 300 volts and the lower platinum plate to the positive 

 pole. The iron plate was then slowly heated with a bunsen. 

 After a short interval the top platinum plate was removed 

 and found strongly radioactive. 



Effects not much differing in intensity were obtained if the 

 plates were uncharged. The only advantage in charging 

 the top plate negatively is to concentrate to some extent the 

 radioactivity on its surface. If the radium was not heated 

 too strongly, the same specimen could be used several times 

 before its power of exciting radioactivity was much dimin- 

 ished. If, however, the radium was heated to a red heat, its 

 power of exciting radioactivity afterwards was to a large 

 extent destroyed. 



Some difficulty was experienced in determining the ab- 

 sorption of the excited radiation due to radium, on account 

 of the rapid diminution of the intensity with time. 



In order to correct for the loss of intensity with time, the 

 rate of leak due to the bare excited plate was taken before 

 and after the substance whose absorption was being examined 

 was placed over it. The observations were taken at regular 

 intervals as rapidly as possible. The mean of the two rates 

 of leak due to the bare plate was taken as a measure of the 

 intensity of the radiation for the intermediate time when the 

 rate of leak due to the excited plate, covered with the absorb- 

 ing substance, was being examined. 



The absorption of the excited radiation due to both thorium 

 and radium is shown in fig. 3 for aluminium foil, in fig. 4 

 for Dutch-metal, in fig. 5 for air at atmospheric pressure and 

 temperature. 



It was found that, for all the substances examined, the 

 absorption of the excited radiation due to thorium compounds 

 was the same as for the excited radiation due to radium, at 

 any rate within the limits of experimental error. This points 

 to the conclusion that these two radiations are the same. 



It is of interest to observe that the power of penetration 

 Phil. Mag. S. 6. Vol. 4. No. 19. July 1902. (J 



