72 SCIENCE AND IMMORTALITY 



In 1898 Monsieur and Madame Curie noticed 

 that pitch-blende, an ore of uranium, had more 

 power of emitting radiations — more radio-activity^ 

 as it is called — than had the salts of uranium ; and 

 after infinite pains they succeeded in obtaining 

 from the ore the substance named radium^ which is 

 about two million times as radio-active as uranium. 

 The radio-activity of radium was carefully and 

 ingeniously investigated, and it was found that 

 the radiations consisted of rays of three kinds : 

 (i) Alpha (a) rays, so-called, consisting of particles 

 about twice the size of a hydrogen atom, charged 

 with positive electricity, and projected at the rate 

 of about 20,000 miles a second ; (2) Beta (fi) rays, 

 so-called, identical with the " cathode " rays of a 

 vacuum tube, and consisting of particles about 

 Y^ the size of a hydrogen atom, charged with 

 negative electricity, and projected at the rate of 

 75,000 to 150,000 miles a second ; (3) Gama (7) 

 rays, so-called, which resemble the X-rays used 

 in X-ray photography, and which have the power 

 of penetrating matter generally considered opaque, 

 e.g. lead, iron. 



Besides uranium and radium, three other mark- 

 edly radio-active substances, thorium, polonium, 

 and actinium, have been discovered. 



Thus, within a hundred years of Faraday's great 

 prophecy, the existence of radiant matter, consisting 



