584 Prof. Rutherford and Mr. Soddy 



It is necessary to consider briefly some of the apparent 

 exceptions to this principle of the conservation of radio- 

 activity. In the first place it will be recalled that the 

 emanating power of the various compounds of thorium and 

 radium respectively differ widely among themselves, and are 

 greatly influenced by alterations of physical state. It was 

 recently proved (Phil. Mag. April 1903. p. 453) that these 

 variations are caused by alterations in the rate at which the 

 emanations escape into the surrounding atmosphere. The 

 emanation is produced at the same rate both in de-emanated 

 and in highly emanating thorium and radium compounds, 

 but is in the former stored up or occluded in the compound. 

 By comparing the amount stored up with the amount pro- 

 duced per second by the same compound dissolved, it was 

 found possible to put the matter to a very sharp experimental 

 test which completely established the law of the conservation 

 of radioactivity in these cases. Another exception is the 

 apparent destruction of the thorium excited activity deposited 

 on a platinum wire by ignition to a white heat. This has 

 recently been examined in this laboratory by Miss Gates, 

 and it was found that the excited activity is not destroyed, 

 but is volatilized at a definite temperature and redeposited in 

 unchanged amount on the neighbouring surfaces. 



Radioactive " Induction" — Various workers in this subject 

 have explained the results they have obtained on the idea of 

 radioactive "induction," in which a radioactive substance 

 has been attributed the power of inducing activity in bodies 

 mixed with it, or in its neighbourhood, which are not other- 

 wise radioactive. This theory was put forward by Becquerel 

 to explain the fact that certain precipitates (notably barium 

 sulphate) formed in solutions of radioactive salts are them- 

 selves radioactive. The explanation has been of great utility 

 in accounting for the numerous examples of the presence of 

 radioactivity in non-active elements, without the necessity of 

 assuming in each case the existence of a new radio-element 

 therein, but our own results do not allow us to accept it. 



In the great majority of instances that have been recorded 

 the results seem to be due simply to the mixture of active 

 matter with the inactive element. In some cases the effect is 

 due to the presence of a small quantity of the original radio- 

 element, in which case the " induced " activity is permanent. 

 In other cases, one of the disintegration products, like uranium 

 X or thorium X, has been dragged down by the precipitate, 

 producing temporary, or, as it is sometimes termed, "false'" 

 activity. In neither case is the original character of the 

 radiation at all affected. It is probable that a re-examination 



