PHYSICAL SCIENCE 47 



with a superficial resemblance to Rontgen rays, but 

 that there was no connection between this effect 

 and the phenomena of phosphorescence. 



M. and Mme. Curie soon observed that certain ores 

 of uranium were more active than their contents of 

 uranium warranted, and, searching for the cause, they 

 gradually separated an intensely active salt, to the 

 metallic base of which they gave the name radium. 



Radio-active substances, of which radium is the 

 most striking example yet discovered, emit at least 

 three kinds of radiation, now distinguished by the 

 Greek letters a, /3, y. The a radiation is easily ab- 

 sorbed by screens. It may, though with difficulty, be 

 deflected from its straight path by magnetic and elec- 

 tric forces, and is thereby shown to consist of posi- 

 tively electrified particles having the mass of helium 

 atoms. The /3 radiation is much more penetrative. 

 It is easily deflected, and shown to be identical in 

 nature with the cathode ray stream. It is made up 

 of negatively electrified projectiles like Thomson's 

 corpuscles, though the /5 rays possess higher velocities, 

 sometimes approaching that of light to within some five 

 per cent. The y rays seem to resemble Rontgen rays. 



Radio-activity is always accompanied by chemical 

 change, and the rate of change indicates that it is 

 of the nature of a dissociation of particles acting singly. 

 The amounts of energy liberated are very much greater 

 than any connected with ordinary chemical actions, 

 and the whole of the evidence suggested to Rutherford 



