CH. XVIIL] AURORA 169> 



form ions, would also serve as centres for condensation 

 of atmospheric water vapour at high altitudes, and so? 

 be liable to affect rainfall. Moreover, the fact that 

 water vapour condenses more readily on negative 

 than on positive ions, seems to furnish us with one ./ 

 explanation of atmospheric electricity ; for a fall of 

 rain would bring down with it a negative charge, 

 and would leave the upper regions positively 

 electrified with respect to the earth's surface : and 

 this agrees with the known sign of the normal field 

 of electric force in the atmosphere. 



These early perceptions have been well elaborated 

 of late by Arrhenius ; and his explanation of the 

 auroraby means of the catching and guiding of 

 rapidly moving electrons by the earth's magnetic 

 lines of force, so as to deflect them away from the 

 tropical sunshine, and to guide them in long spirals, 

 along the lines, to the poles, there to reproduce the 

 phenomena of the vacuum-tube in the rarified upper 

 regions of the atmosphere is particularly definite and 

 pleasing. Some of the other astronomical suggestions 

 he has made are likewise of considerable interest. 



Transformations of Radium, etc. 



The following details are given by Rutherford for 

 the spontaneous transformations which radium under- 

 goes, together with the lifetime constant of the 

 various products that is to say the time required 

 for the activity of the product to fall to one-half its 

 value, and also the kind of particles or rays which 

 are thrown off. 



Certain of the changes are rayless changes, and 

 may be considered to result in an allotropic modi- 

 fication, without change of atomic weight ; but 

 whenever an alpha particle is thrown off, the atomic 



