the Theory of Luminous Flames. 11 



This supposition led to a more exact examination of the 

 changes brought about in the flame of coal-gas by an excess 

 of oxygen. When a flame, burning at the orifice of a wide tube, 

 is placed in an atmosphere of pure oxygen, a notable increase 

 in luminosity takes place within the flame-mantle, which is 

 itself, nevertheless, considerably decreased in size, while the 

 outer non-luminous border of the flame is broadened out. In 

 order to study this action more narrowly, I have found it ad- 

 vantageous to make the flame very small by allowing the gas 

 to issue from a narrow tube. If, for instance, a flame of coal- 

 gas 4 to 5 centims. in length, issuing from a blowpipe-nozzle, 

 be plunged into a reversed jar of oxygen, the appearance of 

 the flame is greatly altered. The outer, scarcely visible, 

 part of the flame increases enormously in size at the ex- 

 pense of the inner and luminous part. A small luminous 

 point alone represents what was formerly a broad luminous 

 band ; at the same time, the whole flame decreases propor- 

 tionately from what it had been in air. This is to be ac* 

 counted for by the absence of diluting nitrogen, a circum- 

 stance which also causes the temperature of the flame to 

 increase considerably. The decrease in luminosity can scarcely 

 be traced to any other cause than the large quantity of 

 pure oxygen, which, by diffusing inwards into the narrow 

 flame, brings about an immediate oxidation of the contained 

 carbon, which is, therefore, not necessitated to spread through 

 the flame in a red-hot state in order to find oxygen sufficient 

 for its combustion. 



If this supposition be true, it follows that decrease of lumi^ 

 nosity can only be brought about by combustion in oxygen 

 in the case of those flames the light-giving constituent of 

 which is capable of being converted by excess of oxygen into 

 a feebly luminous gas, but that those flames the luminosity 

 of which is due to some substance which cannot be transformed 

 by oxygen into such a gas must continue to burn in oxygen, 

 even when issuing from the smallest orifice, with brilliancy— 

 that, indeed, an increase in luminosity must be brought about 

 under such conditions, because of the increased temperature of 

 the flame. 



Direct experiment confirms these deductions, and therefore 

 also the original supposition. 



Hydrogen saturated with vapour of chromium oxychloride 

 (Cr 2 2 Cl 2 ), and issuing from a blowpipe-nozzle, burns in oxygen 

 with a dazzling white light : the luminosity is in this case 

 due to the presence in the flame of chromium oxide. If the 

 hydrogen be laden with the vapour of stannic chloride (SnCl 4 ), 

 it burns, under the same conditions, with a blue flame of much 



