Influence of Temperature upon Luminosity of Flames

The higher the temperature of a flame, the greater will become its luminosity, the change being due to a general shortening of the wavelength of the radiation. This can be readily demonstrated by a comparison of the flames produced by the combustion of cold and heated coal gas respectively; by burning phosphorus or carbon disulphide in air and in oxygen respectively, an atmosphere of the latter gas causing a hotter and brighter flame.

The calculation of the flame temperature for a combustible gas iike hydrogen, carbon monoxide, or methane at first sight appears to be a simple problem since the apparently necessary data are only the heat of combustion and the specific heats of the products. Such calculations always yield very high results much above those recorded by direct experimental measurements. The discrepancy is probably due to a combination of several causes. On account of the temperature of the flame the products are partially dissociated, so that combustion is not complete in the flame. The specific heat of gases increases with rise in temperature, so that the value obtained at the ordinary temperature for the specific heat is too low. In addition to these two causes, another contributory factor is the loss of heat by radiation, which may be very considerable even in non-luminous flames, whilst the general presence of an excess of the " supporter of combustion " and the non- instantaneous character of the combustion also detract from the accuracy of the calculation.

A convenient method of determining the temperature of a luminous flame consists in inserting a thin wire of incorrodible metal, such as gold or platinum, into the flame and estimating the temperature with a pyrometer. The loss of heat occasioned by insertion of the wire is compensated for by raising the wire to the temperature of the flame by an electric current. When the wire has reached the same temperature as the flame there is no deposition of carbon. The wires should not exceed 1 mm. in diameter.