172 PHYSICAL SCIENCE 



effects which depend on one atom coming into 

 action each second. We may well be astonished 

 at the delicacy of this means of research. 



Again, all three kinds of rays produce phos- 

 phorescent and photographic effects, though the 

 penetrating powder of the /3 and 7 rays makes 

 the phenomena due to them more remarkable. 



Radium salts are self-luminous, owing either 

 to the direct emission of light by their agitated 

 atoms, or to some phosphorescent effect of the 

 internal bombardment produced by their radio- 

 activity. The spectrum of this spontaneous 

 luminosity was photographed by Sir William and 

 Lady Huggins, and shown to correspond with 

 the spectrum obtained by passing electric sparks 

 through nitrogen. Sir William Crookes and Sir 

 James Dewar found that this spectrum vanished 

 when the radium compound was placed in a high 

 vacuum. Probably, therefore, it Is due to the 

 effect of the activity of the radium on atmospheric 

 nitrogen surrounding the radium salt or occluded 

 within It. 



A screen of the phosphorescent substance, zinc 

 sulphide, when placed In the neighbourhood of 

 a radium compound, glows brightly, and Crookes 

 has used this property In a most striking and 

 beautiful experiment. A tiny fragment of a 

 radium salt is fixed at the distance of a fraction 

 of a millimetre in front of a plate covered with 

 zinc sulphide. On looking through a lens or 

 a low-power microscope In a dark room, brilliant 

 scintillations are seen, and the effect of the atomic 

 projectiles of the a radiation as they strike the 

 target Is thus made visible to the human eye. 

 In 1908 Rutherford used this effect to count the 



