the Theory of Luminous Flames. 23 



neighbouring pair of molecules likewise to the ignition-point. 

 This action is propagated throughout the mass, and continues 

 so far and so long as the combustible mixture extends. 



Such a process takes place in the combustion of all sub- 

 stances, whether solids, liquids, or gases ; and to this propa- 

 gation of combustion is due the continuity of all flames. 



Now, inasmuch as the gas existing between flame and burner 

 in the cases noticed clearly consists of such a combustible 

 mixture (which may be proved in the case of alcohol vapour 

 by bringing a small flame to the orifice of the tube), it follows 

 that the heat given out by the last pair of molecules actually 

 undergoing combustion must act, in the manner described, 

 upon the pair next them, and so on throughout the gaseous 

 mixture; yet this does not appear to be the case. I say does 

 not appear to be the case, because we are too liable to look on 

 the flame as something having an existence of its own (see 

 Benevides), and not to regard it as a part of the gaseous stream, 

 which is visible to us for a short distance. 



If we may forget for a moment the true nature of the gas, 

 we might compare the burning stream to a rod placed in a 

 fire, which glows in the central parts, the ends emitting no rays 

 of light. In a magnesium lamp the metallic wire is kept in 

 motion by means of clockwork : the position of the flame is 

 thus maintained constant. If the wire is pushed out too 

 quickly or too slowly, the flame is advanced or withdrawn; 

 and a constant position is only maintained by moving out the 

 wire at that rate at which the flame would recede were the 

 wire immovable. 



This recession of the flame is conditioned by the propaga- 

 tion of the ignition ; it becomes more rapid the higher the 

 temperature* of combustion and the lower the temperature of 

 ignition of the combustible body. Thus a stick of phosphorus 

 ignited at one end, and placed in a horizontal position, burns 

 almost at once throughout its entire surface ; a longer period 

 elapses before the ignition of a wick impregnated with petro- 

 leum is propagated throughout the length of that wick ; and 

 if rape-oil be used instead of petroleum, the rate of propagation 

 of ignition is yet slower. 



Besides the difference between ignition- and combustion- 

 temperature, two other points must be noted as conditioning 

 the velocity of propagation of ignition : these are, the specific 

 heat and the conductivity for heat of the burning body *. 



So far as these are concerned, the withdrawal of heat from 

 the issuing gas and admixed air is a circumstance which may 



* The magnitude of the surface and the diameter are of consequence ; 

 but these may he eliminated by parallel trials. 



