14 ROYAL SOCIETY OF CANADA 



The corresponding distance for zinc sulphide was 2.40 — a value 

 intermediate between the other two. 



Since 8 layers of foil are equivalent to 4.3 cms. of air, the ranges in 

 air of the phosphorescent effect for zinc sulphide, barium platino cyanide 

 and willcmite correspond to 6.7, 6.8, and 6.4 cms. respectively. The 

 differences in these values are quite likely due to experimental error. 



Discussion of Results. 



We have seen that the ionizing, phosphorescent and photographic 

 actions of the a rays emitted from radium C cease after traversing 

 very nearly the same distance of air. This is a surprising result when 

 it is remembered that the a particle, after passing through this thick- 

 ness of air, still possesses a velocity of at last 50 per cent of its initial 

 value. Taking the probable value of the initial velocity of the a 

 particle from radium C as 3.5 X 10^ cms. per second, the ionizing, 

 phosphorescent and photograpliic actions cease when the velocity of the 

 a particle falls below 1.5 x 10^ cms. per second, that is, a velocity of 

 about ^^ of that of light. The ])article still possesses nearly 40 per cent 

 of its initial energy of projection at this stage. 



These results show that the property of the a rays of producing 

 ionization in gases, of producing luminosity in some substances and of 

 affecting a photographic plate ceases when the velocity of the a particle 

 falls below a certain fixed value which is the same in each case. It 

 seems reasonable, therefore, to suppose that these three properties of the a 

 rays must be ascribed to a common cause. Now the absorption of the 

 a rays in gases is mainly a consequence of the energy absorbed in the 

 production of ions in the gas. When the « particles are completely 

 absorbed in the gas, the same total amount of ionization is produced, 

 showing that the energy required to produce an ion is the same for all 

 gases. On the other hand, for a constant source of radiation, the 

 ionization per unit volume of the gas is approximately proportional to 

 its density. Since the absorption of the a rays in solid matter is 

 approximately proportional to the density compared with air, it is 

 probable that this absorption is also a result of the energy used up in 

 producing ions in the solid matter traversed, and that about the same 

 amount of energy is required to produce an ion in matter whether solid, 

 liquid or gaseous. 



It is probable, therefore, that the production of ions in the phos- 

 phorescent material and in the photographic film would cease at about 

 the same velocity for which the « particle is unable to ionize the gas. 

 On this view, therefore, the expcriinental results receive a simple 



