Properties of the a Rays from Radium. 175 



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, there- 

 fore, to suppose that these three properties of the ex, rays must 

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

 a rays in gases is mainly a consequence of the energy ab- 

 sorbed in the production of ions in the gas. When the a 

 particles are completely absorbed in any 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 phosphorescent material, and in the photographic film, 

 would cease at about the same velocity for which the a. par- 

 ticle is unable to ionize the gas. On this view, therefore, 

 the experimental results receive a simple explanation. The 

 action of the a, rays in producing photographic and phos- 

 phorescent action is primarily a result of ionization. This 

 ionization may possibly give rise to secondary actions which 

 influence the effects observed. 



This point of view is of interest in connexion with the 

 origin of the " scintillations " observed in zinc sulphide and 

 other substances when exposed to the action of the a rays. 

 This effect is ascribed by Becquerel to the cleavage of the 

 crystals under the bombardment of the a. particles. These 

 results, however, show that we must look deeper for the ex- 

 planation of this phenomenon. The effect is primarily due 

 to the production of ions in the phosphorescent material, and 

 not to direct bombardment, for we have seen that the a par- 

 ticle produces no scintillations when it still possesses a large 

 amount of kinetic energy. It seems not unlikely that the 

 scintillations produced by the a rays must be ascribed to the 

 recombination of the ions which are produced by the a. par- 

 ticle in the crystalline mass. It is difficult to see how this 

 ionization could result in a cleavage of the crystals. 



This close connexion of the photographic and phosphorescent 



