ABSTRACT
Mr. Marsden has shown that the luminosity decreases with continued bombardment to a very small fraction of its initial value. For a given bombardment, the rate of decay of luminosity is about the same for zinc sulphide and willemite, but is especially rapid in barium platinocyanide. The action of the α particles on phosphorescent zinc sulphide is of special interest and importance on account of the marked scintillations observed, and the fact that each a-particle under suitable conditions produces a visible scintillation. Since the amount of impurity present is of the order of 1 per cent., it is probable that only a small fraction of the total number of molecules give rise to the scintillation effect. In the theory of the decay of luminosity outlined in this chapter, it is supposed that a scintillation results from the dissociation of a number of active centres which lie in the path of the α particles.
