Manchester Memoirs, Vol. xlviii. (1904), No. 8. 9 



atoms projected radially into space with terrific energy. 

 The flight of these particles gives rise to the a radiation, 

 their stoppage to all the well-known effects by which the 

 rays may be recognised. Thus fluorescence is produced 

 when the particles strike an obstacle capable of vibrating 

 with such a period as to produce light waves. The ioni- 

 sation of gases is brought about by the collisions of the a 

 particles with the neutral molecules of the gas, whereby 

 the latter are torn apart or dissociated into oppositely 

 charged particles or ' ions.' Most generally and with 

 any obstacle the energy of the a particles is converted 

 into heat. This was shown experimentally last year 

 hy MM. Curie and Laborde, who proved that a solid 

 radium preparation maintains itself some degrees hotter 

 than its surroundings. As we have seen the a rays 

 are stopped with great ease, being completely absorbed 

 by one thickness of paper, and as they are produced 

 •equally throughout the mass of the radio-active substance, 

 this result is to be expected. The radium gets hot through 

 the incessant bombardment to which it is subjected by its 

 own a radiations, in the same way as a target would get 

 hot if placed in the line of fire of a Maxim gun. The 

 real question of course is as to how the a particlp gets its 

 enormous energy in the first place. 



Atomic disintegration would have been a difficult 

 process to establish by direct experiment as the cause of 

 radio-activity, had the phenomenon not been somewhat 

 complex. The complexity of radio-activity, especially in 

 the case of the element thorium, made it possible that so 

 far-reaching a conception as the one put forward should 

 be subjected to rigorous experimental verification. The 

 process of disintegration is analogous in form, at least so 

 far as the individual atom is concerned, to the sudden 

 ■disruption of the molecule of an explosive substance like 



