the Theory of Luminous Flames. 5 



main the same when the mixture is passed over a hot as when 

 it is passed over a cold tube. 



I was therefore compelled to alter the conditions of experi- 

 ment in order to render void that particular effect of the hot 

 tube which has just been described. 



If rise of temperature of the flame is the cause of increased 

 luminosity, the effect must be the same if one heats, not the 

 gaseous mixture, but only the indifferent gas. This experi- 

 ment may be readily carried out with a Bunsen's burner, 

 through the two air-tubes of which are passed small platinum 

 tubes about 7 centims. in length ; the outer openings of these 

 tubes are narrowed so that a quantity of air just sufficient to 

 bring about complete non-luminosity is allowed to enter. 



A thin-walled glass tube, the upper rim of which is covered 

 with platinum, may, with advantage, take the place of the 

 ordinary metal tube of the Bunsen's lamp ; the latter tends 

 to cool the heated air to too great an extent. On lighting 

 tbe gas issuing from the glass tube it burns with a non- 

 luminous flame ; but on strongly heating the two platinum 

 tubes by means of Bunsen's burners (care being taken that 

 the products of combustion do not enter the platinum tubes) 

 the flame becomes luminous. 



It might be supposed that this fact is to be explained on 

 the supposition that the volume of air passing over the plati- 

 num tubes is unaltered by heating these tubes, but that the 

 true quantity of air calculated for equal temperatures is much 

 smaller when the tubes are hot, and that there is therefore a 

 deficiency in the amount of oxygen required to completely 

 burn the carbon, and so to maintain the flame in a non- 

 luminous state. 



In order to show that this supposition is untenable, and 

 that the increase in luminosity is to be ascribed solely to the 

 rise in temperature of the flame, the experiment must be 

 modified. Coal-gas and air, or carbon dioxide, are mixed in a 

 gasometer in such proportions that, when conducted through 

 a platinum tube about 10 centims. in length and 8 millims. in 

 width, the mixture burns with a clear blue flame. If the 

 platinum tube be heated to redness, the flame becomes nearly 

 as luminous as that of ordinary coal-gas. On allowing the 

 platinum tube to cool, the flame again becomes non-luminous. 



From this experiment the conclusion is evidently to be 

 drawn that it is the added heat alone which has caused the 

 flame to become luminous, inasmuch as a diminished supply 

 of air cannot in this case, as in the former, have influenced 

 the result. 



It remains, however, to be investigated whether the gaseous 



