10 Dr. Karl Heumann's Contributions to 



admitting indifferent gases is due to dilution, and also to lower- 

 ing of the temperature of the flame by these gases. 



The fact that a flame which has been rendered non-luminous 

 by means of indifferent gases may be again rendered luminous 

 by heating the tube of the burner, I hope to explain by es- 

 tablishing the following points. 



A flame formed of coal-gas and an indifferent gas or air, 

 and burning blue, requires, in order to cause it to become 

 luminous, a higher temperature than that which is possessed 

 by the luminous undiluted flame. The flame of a Bunsen's 

 burner in which non-luminosity has been brought about by 

 means of air is very hot, but becomes luminous when the 

 temperature is much increased by heating the tube. 



These points in the behaviour of the flame of coal-gas and 

 oxygen, which Wibel adduced in support of his theory, are 

 explained by me as follows. Blochmann and Wibel both 

 noticed that the luminous flame of a Bunsen's lamp, fed with 

 oxygen by one opening while the other is closed, can be rendered 

 non-luminous only by employing a rapid current of oxygen 

 and a cooling surface of metallic gauze, simply because the 

 temperature of the flame, when pure oxygen is employed, is 

 very high. The absorption of heat caused by the entrance of 

 cold oxygen, as also the absolute rise in temperature required 

 by the gaseous mixture in order that it shall become luminous, 

 are entirely, or almost entirely, equalized by the intense heat 

 produced by the combustion in pure oxygen. Therefore the 

 production of non-luminosity is so difficult ; that non-lumino- 

 sity should be brought about only by employing a rapid stream 

 of oxygen and a cooling metallic surface is self-evident. 



It is known that a gas-flame may be caused to burn with 

 great luminosity by the admission in proper quantity, and by 

 a proper method, of pure oxygen. This fact certainly depends 

 upon the production of a very high flame-temperature unac- 

 companied by such dilution as is noticed in the Bunsen's or 

 blowpipe flame when air is employed, and when the diluting 

 gas is nitrogen. In this experiment it is found that the 

 greatest luminosity occurs when a rapid stream of oxygen is 

 introduced, but that too great a quantity of oxygen, as too 

 small a quantity, tends to decrease luminosity. Inasmuch as 

 a much higher temperature might be reached by increasing 

 the quantity of oxygen beyond that at which the maximum of 

 light is evolved, it seemed probable that the actual action of 

 this excess of oxygen in decreasing luminosity was not to be 

 traced solely to its cooling and diluting the burning gas, as is 

 the case with altogether indifferent gases, but that a third 

 cause, perhaps more energetic than either of those just men- 

 tioned, was at work. 



