the Theory of Luminous Flames. 27 



and therefore became equal to the velocity of the issuing gas 

 at a point nearer to the burner than that at which these two 

 velocities were previously equalized. 



The diminution in the distance between flame and burner 

 which was observed to take place in every case when the 

 burner was heated, or when a wire was introduced between 

 the flame and burner, must now be commented upon in the 

 light of the second explanation already detailed. It is easy 

 to understand why the distance in question should be dimi- 

 nished by heating the burner. 



This distance depends upon the difference between the 

 velocities of the gas and of the propagation of ignition ; and 

 the latter is itself a function of the difference between the 

 ignition and combustion temperatures. 



The combustion-temperature is high because of the gas 

 being heated previously to ignition ; the gas has been already 

 heated nearly to its ignition-temperature. These two circum- 

 stances necessarily cause a considerable increase in the rate 

 of propagation of ignition ; the velocity of ignition becomes 

 greater than the velocity of the issuing gas ; and the distance 

 between flame and burner is therefore diminished or entirely 

 removed. The diminution in this distance which is brought 

 about by holding a metallic wire between the flame and 

 the burner, and moving the wire towards the latter, may be 

 thus explained: — The flame-mantle is produced immediately 

 behind the wire because the latter serves to shelter the flame 

 from the cooling influence of the quickly rushing stream of 

 gas. The heat so produced is communicated to the nearest 

 portion of non-ignited gas, and the flame is thus caused to 

 travel backwards towards the burner. 



The familiar phenomenon of the flame of a petroleum-lamp 

 burning above the slit in the piece of thin metal which sur- 

 rounds the wick, is explicable on similar grounds. The flame 

 is so cooled by the metal, at a small distance from the wick, 

 as to be extinguished ; but the lower part of the petroleum- 

 gas still continues to burn. A mixture of unburned petro- 

 leum-vapour and products of combustion of this vapour, 

 therefore passes upwards through the slit. This mixture may 

 be ignited by properly regulating the screw which raises 

 the wick ; but the flame only appears at the distance of a few 

 centimetres above the metallic cap. The velocity of ignition 

 is very small, inasmuch as the combustible matter consists of 

 heavy, easily condensable vapours, which are moreover greatly 

 diluted by the products of combustion of the lower part of 

 the gas, viz. by carbon dioxide and water, substances having 

 high specific heats. The distance between the metallic cap 



