the Method of its Transmission. Ill 



charge of electricity, and are consequently unaffected by the 

 presence of an electric field. 



Jn my first paper on the thorium emanation (loc. cit.) it 

 was pointed out that the particles constituting the emanation 

 certainly did not move with a velocity greater than '00001 cm. 

 per sec. for a gradient of 1 volt, per cm. The conclusion was 

 drawn that the emanation itself was initially uncharged. A 

 similar result is true for the emanation from radium ; for the 

 emanation still persists in a closed vessel after several weeks' 

 exposure in a strong electric field. For these reasons the 

 suggestion made by Becquerel*, that the emanations are 

 composed of positive ions directly emitted from radioactive 

 bodies, is untenable ; for if such were the case, the ions would 

 at once be swept to the electrodes by the electric field, and 

 would very rapidly disappear from the gas. 



These emanations possess the property of ionizing the gas 

 and of producing from themselves positively charged carriers 

 which cause excited activity in bodies on which they are 

 deposited. This property lasts only a few minutes in the 

 case of the thorium emanation, and for several weeks for the 

 radium emanation. 



Two hypotheses may be put forward to account for the 

 origin of these charged carriers : — 



(1) The radioactive matter constituting the emanation con- 

 denses on the positive ions, produced in the gas by the 

 radiation, and is thus transferred to the cathode. 



(2) The particles of the emanation possess the property of 

 expelling from themselves a negatively charged body of some 

 kind. The particle would thus be left with a positive charge, 

 and would be carried to the cathode by the electric field. 



It is not easy to decide definitely between these two hypo- 

 theses, but the evidence as a whole is strongly in favour of the 

 second. 



In regard to (1) it might be supposed that the emanation 

 condensed more readily on the positive than on the negative 

 ion ; on the principle that water and alcohol- vapour condense 

 more readily on the negative than on the positive ion. If 

 this were the case, it would be expected that the emanation 

 would be removed more rapidly if the number of ions were 

 increased in the gas through which the emanation was 

 distributed. There is no evidence that such an effect exists. 

 I have tried the experiment of passing the emanation through 

 a space strongly ionized by radium rays ; but the amount of 

 excited activity in a given time on the cathode, placed in this 



* Comptes Rendu*, Dec. 9, 1901. 



