the Radiations from Radioactive Substances. 13 



Comparing aluminium and air the absorption is thus 

 roughly proportional to the density of all the radiations. 

 The divergence, however, between the absorption -density 

 numbers is large when we compare two metals like tin and 

 aluminium. 



The coefficient of absorption for tin is not much greater 

 than for aluminium, although the density is nearly three 

 times as great. 



The result for the nondeviable radiations is in this respect 

 exactly opposite to that for the deviable uranium rays, for 

 in that case we have shown that the absorption is more 

 than twice as great as would be expected on the absorption- 

 density law. 



Results of this kind point to the conclusion that the 

 mechanism that causes absorption is different for the deviable 

 and nondeviable rays. 



The result of Strutt*, that the relative ionization of hydro- 

 gen compared with that of air and other gases is quite different 

 for the deviable and nondeviable rays of radioactive sub- 

 stance, also supports such a view. 



Emanating Power of Thorium and Radium. 



In a previous paper by one of us it has been shown that 

 thoria gives off a radioactive emanation which behaves in all 

 respects like a radioactive gas. It can be carried along with 

 a current of air, passes readily through tightly packed cotton- 

 wool, and is not appreciably absorbed by bubbling through 

 solutions. The radiating power of the emanation rapidly 

 diminishes, falling to half its value in about one minute. 



Dorn f later found that a preparation of radium from 

 P. de Haen also gave out an emanation which decayed far 

 more slowly than the thoria emanation. 



The specimens of radium used by us gave out very little 

 emanation at atmospheric temperature. 



On heating radium, however, the amount of emanation 

 increased very rapidly. In Phi/sikalische Zeitschrift. April 20, 

 1901, one of us has given an account of the effect of tempe- 

 rature on the emanating power of both thorium and radium. 

 The radioactive substance to be tested was placed in a platinum 

 tube (see fig. 6) through which a slow current of air was 

 passed. The emanation was carried with the air into a metal 

 cylinder, where the rate of leak produced by the emanation 

 was tested by an electrometer in the usual way. 



* Phil. Trans. 1900. 



f Abh. der Nat. Ges. zu Halle, 1900. 



