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 × 109 cms. per sec., the ionizing, phosphorescent, and photographic actions cease when the velocity of the α particle falls below 1·5 × 109 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.