The essential difference between fluorescence and phosphorescence consists in the time during which the light lasts. Fluorescence ceases almost immediately after the exciting cause is withdrawn, while a phosphorescent body whether excited by heat, solar light, or electricity, lasts a much longer time; besides, the fluorescent rays are generally of lower refrangibility. Light and heat are temporarily absorbed and given out again by every body on the surface of the earth, more or less, that are exposed to the sun’s light. The nights would be much darker even when illuminated by the stars were it not for earth light, for the molecules restore to the ether, in the form of phosphorescence, the undulations they have received from the sun’s light during the day. The snow and ice blink of the sailors is a striking instance; generally, however, it is of much shorter duration. The phosphorescent property is nearly allied to electricity, for bodies that are bad conductors are apt to become phosphorescent, while good conductors of electricity rarely if ever show it. Ozone must be phosphorescent, for oxygen exhibits persistent light when electric discharges are sent through it, and Mr. Faraday saw a flash of lightning leave a luminous trace on a cloud which lasted for a short time. In the solar spectrum the chemical or actinic rays produce phosphorescence, which the red rays have the power to extinguish. M. Nièpce de St-Victor found that solar light impresses its vibrations so strongly on substances exposed for a short time to its influence that they not only shine in the dark, but that the phosphorescent light they radiate has chemical energy enough to decompose substances in unstable equilibrium, and leave daguerreotype impressions of great delicacy and beauty.