Radioactivity and Ionization of the Atmosphere. 713 



cannot be ascribed to the presence of a minute quantity of 

 thorium or radium emanations in the atmosphere. The close 

 resemblance in the phenomena, however, renders it probable 

 that the excited activity from the air is due to a process 

 similar in character to that which produces excited activity 

 from the emanations of thorium and radium. One of the 

 authors* has recently shown that in the case of radioactive 

 substances the excited activity is due to a transmission of 

 positively charged radioactive carriers to the cathode. These 

 carriers travel in an electric field with about the velocity of 

 the positive ions produced in air by Eontgen or Becquerel 

 rays. 



There seems to be little doubt that the excited activity is 

 due to a deposit of a minute quantity of intensely active 

 radioactive matter. Such an hypothesis is essential to explain 

 the facts of solutious. and that the radioactivity can be trans- 

 ferred from the radioactive body to the cloth by rubbing. 

 The production of excited activity from the air cannot be 

 ascribed to any surface action on the conductor due to the 

 electric field. A wire does not give any appreciable activity 

 if it is confined in a cylinder where the volume of air is 

 small, although the wire is subjected to the same voltage as 

 in the open air. All the evidence obtained up to the present 

 points strongly to the conclusion that the excited activity is 

 derived from the volume of the air surrounding the charged 

 wire. Since the activity is confined to the cathode, the 

 carriers to which the activity is due must possess a positive 

 charge. These carriers may obtain a positive charge either 

 by the condensation of temporary radioactive matter of some 

 kind round the positive ion already existing in the air, or by 

 the expulsion of a negative electron from the carrier. The 

 latter explanation seems the more probable, for we now 

 knowf that all the radioactive substances thorium, radium, 

 and uranium, as well as the excited activity due to thorium 

 and radium, possess the property of spontaneously expelling 

 electrons. 



There is as yet no definite evidence of the origin or mode 

 of production of these radioactive carriers in the air, but 

 assuming their presence, many of the experimental facts 

 observed receive a simple explanation. 



The higher the potential of the wire the greater the distance 

 from which the carriers are conveyed to the cathode. The 

 amount of excited activity on a wire exposed in free space, 

 on this view, should increase rapidly with increase of voltage. 



* Phys. Zeit. x. p. 210 (1902). 



t Rutherford and Grier, Phys. Zeit. xvii. p. 885 (1902). 



