the Theory of Luminous Flames. 29 



sides of the metallic cap ; at the same time the flame-tempe- 

 rature is increased, and the motion of the heated particles 

 of gas is accelerated. These circumstances act in opposition 

 to the cooling effect of the metallic cap. 



These experiments may be interpreted as pointing to the 

 withdrawal of heat from the sides of the stream of gas and 

 air as the cause of the space noticed between flame and burner ; 

 but it has been shown that this action is but small, and that 

 the superior velocity of the stream of gas over that of the 

 propagation of ignition is the principal cause of the observed 

 effect. Whether this be the sole cause cannot be determined 

 until further experiments have been carried out. 



The most important points established in the foregoing 

 part of this paper may be summarized thus : — 



1. The fact that a gas-flame does not rest upon the burner 

 nor a candle-flame upon the wick, as also the fact that a flame 

 never directly touches a cold body held within it, is to be 

 explained by the cooling action exercised upon the gas by its 

 surroundings. 



The combustible gases are cooled throughout a definite 

 space below their ignition-temperature ; the flame is there- 

 fore extinguished. This conclusion is opposed to that of 

 Blochmann. 



2. The very considerable distance noticed between the 

 burner and the flame of a gas issuing under high pressure, 

 or mixed with a large volume of an indifferent gas, cannot be 

 accounted for on the grounds put forward by Benevides. 

 The production of such a distance is much rather to be traced 

 to the cooling action of the stream of gas and of the outer 

 air, and perhaps more especially to the fact that the velocity 

 of the stream of gas in the neighbourhood of the burner is 

 greater than the velocity of propagation of ignition within 

 the gas. 



3. In order that other circumstances conditioning the effect 

 may be removed, the velocity of propagation of ignition must 

 be equal to that of the gas-stream at the point, situated some 

 distance from the burner, where the flame begins. 



Determinations of the velocity of ignition should be made 

 under these conditions for different gases ; and since this 

 magnitude is a function of the difference between ignition 

 and combustion temperatures, conclusions may be drawn from 

 such experiments regarding the relations existing between 

 these points *. 



* Since going to press, I have noticed an interesting paper by E. 

 Mallard [Annates des Mines, 1875, iii. 355], in which the velocity of 



