CH. XVIIL] SPECTRUM OF RADIUM 



175 



disturbed by the neighbourhood of radium, to the 

 extent of emitting luminous radiation without eleva- 

 tion of temperature. The spontaneous radium -glow 

 is, in fact, probably due to its influence on other 

 substances ; and ordinary glass, exposed to it, darkens 

 and becomes thermo-luminescent, that is to say, it 

 begins to emit light when raised to a temperature 

 of about 500 degrees. 



M. Eugene Ne'culcea gives the following table of 

 the photographic spectrum of the radium spark, as 

 observed by M. Demargay, the wave lengths being 

 given in " tenth-metres." In the visible spectrum, 

 which is not photographed, there is only one notable 

 ray, of wave-length 5665. For other observations^ 

 see Runge, Astrophysical Journal, 1900, p. 1. 



The detection of radium by the spectroscope, 

 through its strongest line, 38147, though it may be 

 a method perhaps a million times more sensitive than 

 ordinary chemical analysis, has been shown to be a 

 million times less sensitive than a method of detec- 

 tion by means of an electroscope, utilising the 

 extraordinary ionising power of its radio-activity. 

 For Rutherford reckons that each a particle expelled x 

 from radium is able to generate some hundred thou- 

 sand ions before it is stopped, or rather before its 



