the Theory of Luminous Flames. 91 



hypothesis (I shall endeavour to bring together further proof 

 of this hypothesis in the succeeding parts of the present 

 paper) : — 



Carbon-containing sources of light may burn with luminous 

 flames, i. e. with separation of carbon in the flame, or with non- 

 luminous flames, i. e. without separation of carbon*. The 

 maintenance of a certain {high) temperature, dependent upon 

 the nature of the combustible substance, is an essential condition 

 of luminosity; aflame ivhose temperature has been lowered by 

 any means is no longer able to bring about the required separa- 

 tion of carbon. Combustible matter, when diluted with indif- 

 ferent gases, requires to be maintained at a higher temperature 

 in order that it may burn ivith a luminous flame than when it is 

 undiluted with such gases. 



Chemistry furnishes us with many reactions analogous to 

 the last-mentioned circumstance : thus dilute solutions gene- 

 rally undergo decomposition or throw down precipitates only 

 when strongly heated, &c. 



The various parts of a luminous flame are possessed of very 

 varying temperatures ; a cold object brought into different 

 parts of the flame, and withdrawing nearly equal amounts of 

 heat from each, will cool the upper hot parts only slightly 

 below that point at which deposition of carbon takes place ; 

 this process will therefore continue all around the cold object. 

 In such a case as this, separation of carbon will also be taking 

 place at a point in the flame lower than that where the cold 

 object is situated ; hence these little particles of separated 

 carbon will rise upwards, and some of them will be deposited 

 on the cold object. But if this object be placed in the lower 

 and cooler part of the flame, the temperature of this part of 

 the flame will be decreased beyond that point at which carbon 

 is separated ; hence the object will not be covered with a layer 

 of soot : and, from the conditions of the experiment, there can 

 be no separation of carbon in parts of the flame underneath 

 the point where the cold object is now placed. 



Bearing these points in mind, we shall be able to trace the 

 phenomena noticed in the first part of the experiment with 

 the porcelain rod to two circumstances : — 



1. Deposition of carbon was noticed upon the rod when 

 held in the upper part of the flame, because the cooling action 

 of the porcelain was not sufficient to reduce the temperature 

 of this part of the flame below the point at which carbon is 

 separated. 



* I strenuously uphold the old idea that the luminosity of carbon-con- 

 taining flames is to be traced to the presence of separated carbon. In a 

 future paper I will bring forward new proofs of the truth of this idea. 



