14 Dr. Karl Heumann's Contributions to 



ducts of combustion, and also by lowering of the temperature 

 of the flame. 



These two causes gradually increase, until the flame, which 

 continually increases in size, becomes blue, then invisible, 

 and finally, being cooled below the point of ignition, goes 

 out. 



Besides cooling and dilution of the carbon-containing flame, 

 a third cause has been shown to influence the decrease of 

 luminosity — viz. the energetic destruction of the luminous 

 material, i. e. the^oxidation of carbon to feebly luminous gases 

 (carbon monoxide and dioxide). 



Generalizing the results of the experiments upon the means 

 by which flames which have become non-luminous may be 

 again restored to luminosity, we find : — 



1. That hydrocarbon flames which have lost their luminosity 

 by withdrawal of heat again become luminous by the addition 

 of heat. 



2. That flames rendered non-luminous by dilution with air 

 or indifferent gases become luminous by raising their tempe- 

 ratures. 



3. That flames rendered non-luminous by excess of oxygen, 

 which brings about energetic oxidation of the carbon, are 

 again rendered luminous by diluting the oxygen with indifferent 

 gases* 



It Would be very interesting to observe whether flames 

 rendered non-luminous by admixture of indifferent gases may 

 be again rendered luminous by heating the tube of the burner, 

 the combustion being carried out under such pressures as 

 would cause the molecules of the burning gases to maintain 

 their original proximity to one another, notwithstanding the ad- 

 mixture of nitrogen or carbon dioxide. I have not myself the 

 necessary apparatus at hand ; but I would direct the attention of 

 any chemist who is interested in these experiments upon lumi- 

 nous flames to the subject. It would also be well to note 

 whether the decrease in luminosity suffered by a small gas-flame 

 when burned in oxygen is maintained when the oxygen is 

 diluted to one fifth or further. 



If, in the case of previous observers, the point of dispute 

 was whether cooling or dilution were the cause of decreased 

 luminosity in carbon-containing flames when the combustible 

 material was mixed with air or indifferent gas, to me it 

 appears that there are at least three causes, each of which is 

 capable of decreasing the luminosity of these flames, viz. 

 withdrawal of heat, dilution, and oxidation of the luminous 

 material. 



In most cases two or all of these causes are at w r ork : — in 



