112 Prof. Rutherford on Excited Radioactivity and 



space, was not appreciably altered. I have also tried. expe- 

 riments to see if the radiation from the emanation was affected 

 by exposure in a strong electric field, but with negative 

 results. In order to test this, the thorium was placed in the 

 bottom of a small lead box, and covered with two layers of 

 paper to cut off the direct radiation. The top of the box was 

 tightly covered with a very thin layer of mica. This pre- 

 vented the escape of the emanation, but allowed the radiation 

 from the emanation to pass through and ionize the gas above 

 the vessel. The amount of this ionization outside the vessel 

 was unchanged if the emanation was exposed to a strong- 

 electric field by charging insulated conductors placed inside 

 the vessel. 



If the emanation is removed to the cathode in an electric 

 field by condensation on the ion, it is to be expected that it 

 would continue to radiate at the same rate on the electrode as 

 in the gas from which it is removed. On this view the 

 radiation from the cathode should rapidly decrease for the 

 first few minutes after removal from the emanation. Some 

 special experiments were tried to settle this point, but no 

 decrease was observed, although even a minute effect could 

 have been readily detected. 



There is thus considerable indirect evidence against the 

 condensation hypothesis ; and it has consequently been dis- 

 carded in favour of (2), which offers a satisfactory expla- 

 nation not only of the production of positive carriers, but 

 also of the origin of the radiation given out by the emanation 

 itself. On this view the emanation consists of matter in an un- 

 stable state, which undergoes further chemical change. The 

 change consists in the expulsion of a negative particle from 

 the neutral molecule. The residual portion of the molecule 

 retains a positive charge, and is carried at once to the cathode 

 in an electric field. This matter again undergoes chemical 

 change, giving rise to the phenomena of excited radioactivity. 

 The experimental data in favour of this view are best consi- 

 dered in the next section (§ 10) on the nature of the radiations. 



It has been shown that the carriers of excited activity for 

 both thorium and radium travel at about the same rate as the 

 ions produced in the air by Rontgen rays. From data of 

 the Kinetic Theory of Gases it has been shown that the ion in 

 air is probably large compared with the molecule of oxygen 

 or hydrogen. This has been explained by supposing that the 

 ion, immediately after its production, becomes the centre of a 

 cluster of molecules which move with it. On this view that part 

 of the emanation molecule which retains a positive charge im- 

 mediately becomes the nucleus of an aggregation of molecules 



