i8o PHYSICAL SCIENCE 



by the presence of slight traces of some active 

 emanation in the atmosphere, and the consequent 

 active deposit on solid materials, which active 

 deposit is removed by cleaning. Nevertheless, 

 it seems that a few elements such as potassium, 

 not classed as radio-active, show the effect to an 

 extent just measurable. 



The air of the atmosphere itself, when tested 

 with a sensitive electroscope, is found to possess 

 a slight conductivity. It seems likely that this 

 effect is due to traces of some radio-active sub- 

 stance, whence issue the radiations which ionize 

 the air. The rate of leak of electricity through 

 air has been shown by Elster and Geitel to be 

 greater in a cave or cellar than in the open ; 

 while air drawn from a clay soil contained a 

 radio-active emanation. From such experiments 

 we know that traces of some radio-active sub- 

 stance are present in many places in the earth ; 

 on the other hand, we know that some active 

 bodies emit radiations of an extremely penetrating 

 nature. It thus seems reasonable to believe that 

 the slight conductivity which appears to exist at 

 all times in the atmosphere is due to the pro- 

 duction of gaseous ions by the action of stray 

 radiations proceeding from some radio-active 

 material, near or far. 



It was hoped at first that radium might play 

 a useful part in the curative treatment of certain 

 diseases. Rontgen rays have occasionally been 

 employed as a means of checking the spread of 

 cancer, and the radiations from radium also 

 appeared to be effective, besides being applied 

 far more easily locally, and for considerable 



