M. Giesel has made a preparation of barium-radium platinocyanide.

Suddenly he noticed a flicker on a barium platinocyanide screen.

The changed barium platinocyanide is partially regenerated by the action of light.

When this salt is newly crystallised, it has the appearance of ordinary barium platinocyanide and is very luminous.

But if the whole be exposed to light, the platinocyanide is partially regenerated, and if the whole is replaced in darkness the luminosity reappears with vigour.