454 Prof. V. B. Lewes. The Cause of Luminosity [Mar. 21. 



whilst if all were compared when burning from flat- flame burners 

 of the same size as those in which the temperatures were determined, 

 the results when calculated to a consumption of 5 cubic ft. an hour 

 would be 



Acetylene 211-0 



Ethylene 31'5 



Coal gas nil 



Here then we have the anomaly of three gases, which not only do 

 not conform to the preconceived expectation, but which have their 

 ratio of temperature and illuminating value directly opposed to each 

 other. 



In the case of the acetylene and ethylene, moreover, the molecules 

 contain the same number of atoms of carbon, and yet we obtain so 

 enormous a discrepancy in their illuminating value. 



The fact that there is no apparent relation existing between the 

 temperature of the flame, or the probable number of carbon particles 

 contained in it and its illuminating value, at once suggests that the 

 luminosity must be in great part governed by some ther mo-chemical 

 changes taking place in the flame itself, and which do not of necessity 

 affect the average temperature of the flame to any great degree. 



The researches of Hittorf* and Siemens show that air, steam, and 

 the oxides of carbon, even when heated to temperatures above those 

 existing in luminous hydrocarbon flames, are perfectly non-luminous, 

 and the fact that the Bunsen flame, when supplied with sufficient air, 

 has a temperature exceeding 1800 C. in its hottest part, and yet 

 emits no light, shows us that it is exceedingly unlikely that any 

 interactions leading to luminosity take place amongst these ordinary 

 flame gases. 



The fact that most of the unsaturated hydrocarbons in the flame 

 are converted into acetylene before luminosity commences, naturally 

 draws one's attention to this body, and the fact that it is highly 

 endothermic, at once suggests the idea that it may be the liberation 

 of heat during its decomposition that endows the carbon particles 

 produced from it with an incandescence far higher than any which 

 could be expected from the temperature of the flame. 



Berthelot has calculated that the temperature developed by the 

 detonation of acetylene at constant volume is no less than 6220 C., 

 and if this be imparted at the moment of its liberation to the products 

 of its decomposition, the incandescence of the carbon particles is at once 

 explained. 



If luminosity be even partly due to this cause, the detonation of 

 pure acetylene first recorded by Berthelot should develop light. In 

 order to see if this were so, a thin glass tube, closed by a cork, had 



* ' Wied. Ann.,' vol. 7, pp. 587, 591. 



