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A flame is a gas in combustion - the consequence of a chemical reaction in which a gas combines with oxygen in the air. In doing so, energy is released which is sensed as heat and light.
Other than by changing its temperature, the colour of a flame can be changed by introducing into it a material containing a chemical element. For each different element used, a different (characteristic) colour can be obtained. The introduction of sodium imbues the flame with a pure yellow appearance, and can be observed by sprinkling common salt (sodium chloride) into a household gas flame. Potassium gives a purple flame, lithium and strontium give red, copper and tellurium generally give greens, and copper chloride gives blue.
Certain metals can also be heated by forcing a powerful electrical current through them. In the traditional household bulb, a sealed glass globe is fastened at its base into a metal bayonet (UK) or screw-cap (US) socket. Inside the globe, glass stems support a fine coil-filament of tungsten thread, which completes an electrical circuit between two electrodes. When carrying an electric current, the tungsten is heated to incandescence and passes rapidly through 'red' to 'white heat' (at approximately 3000K). Oxygen is evacuated from the glass globe and replaced by an inert gas (commonly a mixture of argon and nitrogen) which delays the evaporation of the tungsten wire and secures a long economic life, commonly of 1,000 hours.
By introducing a small quantity of a halogen gas, it is possible to set up a regenerative cycle, which prevents evaporated tungsten depositing itself on the inner surface of the globe. This may double the useful life of the bulb and also stabilises its colour rendering properties. Tungsten halogen lamps are therefore preferred for use in film projectors, photographic enlargers and automobile headlamps.
For many centuries, the only light sources that were available were flame sources from vegetation, fatty oils or, from about 500 C.E., candle-wax from tallow. The gas lamp, invented by Dixon (1760), applied by Richard Murdoch (1792), and vastly improved by Auer von Welsbach's mantle (1866), was the principal source of artificial illumination throughout the nineteenth century. |
