Take some thick oyster shells, wash them, and calcine by keeping them red hot in an open fire for half an hour: then, select the clearest and whitest parts, and reduce them to powder. Mix three parts of this powder, with one of the flour of sulphur; fill a crucible with this compound, pressing or beating it down as hard and solid as may be, without breaking the crucible. Set the crucible in the fire, and heat it moderately at first, but increase the heat gradually for an hour, in which time it must approach nearly to a white heat. Then let it cool, and again select from the mass, the whitest and purest parts, which must be preserved in a phial with a glass stopper. This powder has the peculiar property of imbibing the rays of the sun in the day time, and emitting them again in the night; or if the phial containing it, be exposed for a few minutes to the direct rays of the sun and then carried into a dark room, light enough will be evolved to render it distinctly visible.