The corresponding distance for zinc sulphide was 2·40 cms., a value intermediate between the other two.
Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges in air of phosphorescent action for zinc sulphide, barium platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms. respectively. The differences observed are quite likely to be due to experimental error.
Discussion of results. We have seen that the ionizing,
phosphorescent, and photographic actions of the α 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 α particle,
after passing through this depth of air, still possesses a velocity of at
least 60 per cent. of its initial value. Taking the probable value of
the initial velocity of the α particle from radium C as 2·5 × 10^9 cms.
per sec., the ionizing, phosphorescent, and photographic actions cease
when the velocity of the α particle falls below 1·5 × 10^9 cms. per second,
that is, a velocity of about 1/20 of that of light. The particle still
possesses nearly 40 per cent. of its initial energy of projection at this
stage.
These results show that the property of the α rays of producing ionization in gases, of producing luminosity in some substances, and of affecting a photographic plate, ceases when the velocity of the α 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 α rays must be ascribed to a common cause. Now the absorption of the α rays in gases is mainly a consequence of the energy absorbed in the production of ions in the gas. When the α 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 α rays in solid matter is approximately proportional to the density of the absorbing medium 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
