Certein bodies... become luminous when heated. Their luminosity disappears after some time, but the capacity of becoming luminous afresh through heat is restored to them by the action of a spark, and also by the action of radium.

Summary

This quote highlights the phenomenon of certain bodies becoming luminous when subjected to heat. Although their luminosity fades over time, they can regain their capacity for luminosity through either the action of a spark or the presence of radium. It suggests that these bodies need external triggers or sources of energy to reawaken their ability to emit light.

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