Luminosity

Luminosity is produced when any body is sufficiently heated. A ball of iron will emit light if its temperature is about 1300° O, and it then looks white hot. The temperature required to make a gas luminous is extremely high, and flame (q.v.) is then produced. It often happens that a flame gives very little light - a spirit lamp is an example of this - but the introduction of something solid into the flame at once increases its luminosity. Except in a very few cases, solid particles of some Idnd must be present in order that a flame shall give light. The luminous part of a gas flame contains countless particles of solid unburnt carbon, but if air is supplied to the burner so that these particles are burnt up. the flame becomes almost invisible. The luminosity of a flame is increased if the temperature can be raised, when, for example. a substance is burnt in oxygen instead of air. It is also increased if the surrounding atmosphere can be condensed. A spirit lamp has been found to give a bright light when burnt in compressed air.