the Theory of Luminous Flames. 13 



general weakening of the light until complete non-luminosity 

 is attained. The flame then increases in size, and finally goes 

 out. 



This kind of non-luminosity exhibits a great resemblance 

 to that noticed when the burning material is diluted by 

 mixing with it indifferent gases, such as carbon dioxide and 

 nitrogen ; the causes of non-luminosity are indeed in both 

 cases identical. Inasmuch as every ordinary flame (with the 

 exception of the flames of explosive substances) requires for 

 its existence two combustibles, the chemical union of which 

 brings about the glowing of the gases, it follows that it is a 

 matter of indifference which of the combustible materials is 

 diluted by indifferent gases — the coal-gas for example in 

 Knapp's experiments, or the oxygen of the atmosphere. 



And in fact it may be shown that a gas burning with 

 luminosity in ordinary air, burns with a blue flame when 

 plunged into a mixture of 5 volumes of air with 2 volumes 

 of carbon dioxide. This experiment is the converse of Knapp's ; 

 and, as in that case, decrease of luminosity is due to dilution, 

 and cooling of the flame. Instead of diluting the air with 

 carbon dioxide previously to the experiment, the products of 

 combustion may be allowed themselves to bring about this 

 dilution, the gas being burned in an inverted globe : it is 

 then noticed that the flame quickly becomes less luminous 

 and then burns blue, at the same time increasing in size. 



The flame remains non-luminous and yet large, but again 

 becomes luminous if transferred at the proper moment to the 

 atmosphere ; otherwise it goes out. 



This experiment on decreased luminosity is perfectly ana- 

 logous to that described in the case of a flame burning in 

 oxygen ; only in this instance nitrogen was absent, and the 

 products of combustion were the sole diluents of the 

 oxygen. 



When a small gas-flame is plunged into an inclosed volume 

 of oxygen there is noticed, then, 



1. Decrease in luminosity of the flame, accompanied by 

 increase in the size of the flame, the light from which is very 

 small; 



2. Increase in luminosity commences, and proceeds until 

 the flame exhibits an appearance similar to that which it 

 possesses in ordinary air, because the energetic oxidizing action 

 of the pure oxygen upon the glowing matter in the flame is 

 moderated by the diluting products of combustion ; 



3. A general decrease in luminosity ensues, but now by a 

 lowering of the intensity of light of the whole flame, brought 

 about by the increasing dilution of the oxygen by the pro- 



