THE luminescence associated with the wetting of finely divided solids has already been reported1–3. With greatly improved arrangements for heating the solid, redistilling the wetting liquid and wetting the solid in the same vacuum, it has now been found that silica from various sources heated in vacuo to 700° C. for a week retains some of its blue luminescence. In dry air or oxygen, heating at 650° C. or more for not less than twenty-four hours completely destroys its luminescent efficiency. Subsequent heating in pure dry hydrogen to 300° C. or above for a few minutes gives again brightly luminescent products the efficiency of which increases to a saturation value with time and temperature of heating. At room temperature hydrogen does not restore the luminescent efficiency destroyed by previous heating in air, so that presumably a chemical reaction is responsible for the effect at higher temperatures. None of the samples heated in hydrogen showed an afterglow of visibly appreciable time of decay except those heated for not more than ten minutes to 300° C. This indicates a concentration quenching of afterglow.