the Cause and Nature of Radioactivity. 583 



was repeated under the saine conditions, using the same 

 sample of nitrate, the compound behaved normally. It thus 

 appears that the emanation can be almost entirely prevented 

 from escaping in the radioactive state in some cases, and 

 partially prevented in others, where no visible peculiarity of 

 physical condition exists, and where other preparations 

 similarly prepared behave normally. 



These are outstanding points in the theory which remain 

 to be explained. It is not possible at present to decide 

 whether these variations of emanating power are caused by 

 an alteration in the velocity of the reaction which produces 

 the emanation, or by an alteration in the time taken for the 

 latter to escape. The experiments detailed in the first paper 

 on the augmentation of the proportion of excited activity in 

 compounds de-emanated by ignition appear to favour the 

 view that the change still proceeds, but the emanation does 

 not succeed in escaping. The experiment on the regular 

 variation of emanating power with temperature might be 

 explained quite well by either hypothesis. 



VIII. The Excited Radioactivity from Thorium. 



Since the emanation gives rise to the phenomenon of 

 excited radioactivity, and the latter appears to be caused by 

 an intensely active invisible deposit of matter, it must be 

 supposed that a tertiary change is taking place. The emana- 

 tion, a gaseous product of the secondary reaction, is again 

 changing and giving rise to a third reaction-product causing 

 the excited activity. The fact that it is manifested entirely 

 on the negative electrode in an electric field, points to the 

 positive ion being the means by which it is transported. 

 Without, in the present paper, going further into the con- 

 sideration of excited radioactivity, it may be mentioned that 

 the successive changes occurring in the thorium atom are not 

 yet ended at this stage. The fact that the excited radiation 

 consists in part of cathode-rays may be recalled here. Further, 

 the intensity of excited activity at first increases from the 

 time of its formation, exactly as in the case of ThX newly 

 separated from thorium, the increase reversing the effect of 

 the normal decay. The radium excited activity behaves in a 

 somewhat analogous manner. The matter in this case 

 causing excited activity does not appear to be homogeneous, 

 but behaves in its action towards acids, &c. as if consisting 

 of two different kinds (compare Rutherford, Pliys. Zeit. 

 p. 251, 1902). 



