LUMINOSITY. The luminosity of flames, such as those obtained by burning coal-gas or a candle, is due mainly to the' incandescence of the carbon particles produced by the chemical decomposition of the combustible gases. If enough air is mixed with the latter previous to their entering the flame, complete combustion takes place as soon as the hot part of the flame is reached; there is, eonsequently, no separation of solid carbon, and the flame is practically non-luminous throughout. This is the principle of the well-known Bunsen burner. That solid carbon exists in an ordinary luminous flame may be readily shown by hold ing in it a glass rod or a piece' of white porce lain, a coating of carbon being then deposited in the form of soot. A certain amount of light, however, is also produced by the in candescence of the gases themselves; this amount of light emitted is proportional to the ca pacity of the gases for absorbing the same kind of light at the temperature of the flame. The
luminosity of a flame depends greatly on its temperature, and increases even more rapidly than the latter; the luminosity of ordinary flames is considerably diminished by the dissociation of carbonic acid, since the dissociation causes a fall of temperature. Another one of the factors de termining the luminosity of a flame is the den sity of the medium in which combustion takes place, an increase of density usually causing a corresponding increase of luminosity. Thus in air kept under a constant pressure of about 10 atmospheres, hydrogen burns with a bright flame; on the contrary, under sufficiently diminished pressures a candle may burn with a pale, non luminous flame. The size of a flame depends on the pressure, the temperature, and the diffusi bility of the reacting substances, on the form of burner used, etc.