The Romance of Modern Chemistry

PRODUCTION OF LIGHT AND HEAT The flame is very hot, gives out almost no light, and if a cold surface is put into the flame, no soot is deposited. This kind of flame is therefore especially suitable for heat- ing and cooking purposes, for blackening of the utensils is avoided. The part played by the air in such a burner can be very simply demonstrated. If the burner is lighted and the observer puts his fingers over the air inlet holes at the bottom of the tube, the flame, instead of giving practically no light, becomes luminous at once. If the reader will take the trouble, this little experi- ment may be carried out with an ordinary incandescent burner. The air inlet holes are easily discovered, and if the burner is lit on some occasion when the mantle has been removed, the effect of letting in or shutting off the extra supply of air is very evident It has been already stated that an ordinary gas or candle flame is luminous because it contains particles of unbumt carbon which are raised to incandescence, and so emit light. If this is so, then we may expect that if we take a non-luminous flame like that of a Bunsen burner, and introduce into it some solid substance which can stand a very high temperature without melting, this flame will become a source of light This is exactly the principle which has been applied in our modern incan- descent burners. As has just been pointed out, the flame of an incandescent burner, apart from the mantle, is quite without luminosity, and the mantle is simply an infusible substance which is raised to incandescence by the heat of the flame. A similar device used to be much in vogue for the exhibition of lantern slides—in the so-called lime-light. By allowing a very hot flame to play on a little lump of 113 h