the Theory of Luminous Flames. 25 



an explosive mixture of carbonic oxide and oxygen C was 

 equal to less than 1 millim. per second. 



These numbers cannot be made use of in the determination 

 of the rate of propagation of ignition of a gas burning in air, 

 inasmuch as in this case the admixed nitrogen and the ab- 

 normal conditions under which the combustible gas is mixed 

 with air greatly diminish the rate of propagation. 



In order to render clear the relations existing between the 

 rates of issue and of propagation of ignition in the case of rapid 

 gas-streams, the following considerations will be serviceable. 



A burning gas obeys the law that the position of the base 

 of the flame remains fixed when the rate of propagation 

 of ignition is equal and opposite to the rate of issue of the 

 gaseous stream. The latter is greatest close to the orifice of 

 the burner, and decreases as this point is receded from, because 

 of the opposition offered by the surrounding air. At all 

 points where the velocity of the gas is greater than the velo- 

 city of propagation of ignition, the flame cannot exist of 

 itself, because each gas-molecule will be carried to a point 

 further than that to which the ignition is transmitted in the 

 same time. 



If, on the other hand, the rate of propagation of ignition is 

 greater than the rate at which the gas-stream moves, the base 

 of the flame will be driven back against the burner and will 

 remain stationary at that point where the two velocities are 

 exactly equal. 



If a burning body be brought into the stream of gas, issuing 

 under high pressure, at a considerable distance from the burner, 

 the flame which is produced moves back against the stream 

 of gas until it reaches the point defined above, where it re- 

 mains stationary ; if, however, the gas be ignited at the 

 opening of the burner, the flame is carried along with the 

 stream until the same point is reached. If the velocity of the 

 gas-stream be increased, the flame moves further from the 

 burner; if the velocity be diminished, the flame approaches the 

 burner; and the flame rests quietly upon the burner only when 

 the two velocities are equal, or when the velocity of propaga- 

 tion of ignition is greater than that of the issuing gas. The 

 last-named condition holds in our ordinary luminous flames, the 

 small distance generally noticed between flame and burner, 

 or wick, resulting from the cooling action of the surroundings 

 of the flame. The phenomena just described may be noticed 

 in the flame of alcohol vapour issuing from an orifice with 

 considerable velocity, as already described. If air be blown 

 through benzol and a light be then brought to the mixture, 

 a flame is produced which moves backwards or forwards as 



