Incandescence

Incandescence, the condition of a body when, by reason of its temperature, it emits white light. As the temperature rises the molecular motions become more rapid and give rise to waves in the surrounding medium of greater frequency. At a temperature of about 400° C. the waves emitted are of sufficiently high frequency to have a slight effect upon the eye. The substance then appears of a faint grey colour, which changes to dull red, bright red, orange, yellow, white, and violet, as the temperature is continually raised. A substance at violet-heat, like the sun, is not only giving out violet light, but also light of all colours corresponding to a lower temperature - a fact that is proved by the ordinary experiment of analysing sunlight with a glass prism. High temperature is not alone sufficient to produce brilliant incandescence; a pure hydrogen flame is almost colourless, but it will easily render platinum wire white-hot. The electric arc is much less brilliant than the glowing carbon poles, though ata much higher temperature. In fact, the presence of solid matter is almost essential; not always so, as the incandescent air of a flash of lightning proves. Incandescent lamps have filaments of specially-prepared carbonaceous material, enclosed in glass vessels from which the air has been removed as effectually as possible. The ends of the filaments are connected to two terminals on the outside of the lamp by means of platinum wires passing through the glass, and a current of electricity is sent through the lamp by connecting up the circuit with the terminals. The high resistance of the lamp causes a great rise of temperature of the filament, which quickly becomes incandescent. If there be air inside the lamp the filament is burnt away. If the current is too strong the temperature becomes extreme and the filament bursts. A body may give out light without being at a high temperature itself; a glowworm is an example of such a body, but the explanation of this and of the few similar exceptional cases has not yet been given.