the Theory of Luminous Flames. 19 



tion of F. Benevides*, the flame of strongly compressed coal- 

 gas allowed to issue into the air, is separated by a space of 

 several centimetres from the orifice of the tube whence it 

 issues. If the pressure amount to two atmospheres, and the 

 tube be 45 centims. in length and 4 to 9 centims. in width, 

 the distance between the orifice of the tube and the flame 

 amounts to about 4 centims. Benevides found the tempera- 

 ture of the dark space to be very low, which is only what one 

 would expect. 



The same author noticed that a flame brought near to the 

 dark space was carried along by the stream of gas. This he 

 regarded as proof of the dilution of the gas with air, caused 

 by the surrounding atmosphere being carried along with the 

 gas-stream which issued from the exit-tube with considerable 

 velocity. If a wire be placed in the flame and be moved 

 backwards through the dark space, the flame also moves back- 

 wards towards the burner, but returns to its original position 

 immediately the wire is removed. 



Benevides looks on these facts as justifying the conclusion 

 that the formation of the dark space is due to the mechanical 

 action of the issuing gas, whereby the air is driven aside for a 

 certain distance from the orifice of the exit-tube ; in this space 

 the requisite amount of oxygen is therefore not obtainable by 

 the gas, which consequently remains unburned. If the exit- 

 tube be very narrow and the velocity of the issuing gas be 

 great, the pushing back of the air may become so intense as 

 to render combustion impossible ; the flame is therefore extin- 

 guished. 



I cannot profess to be satisfied with these explanations. I 

 cannot yet understand how the existence of the flame becomes 

 impossible on the ground that the oxygen is driven back by 

 the gas, and at the same time that the flame is extinguished 

 through want of oxygen. Such a condition is found in the 

 interior of every ordinary flame, not in the flame of compressed 

 coal-gas only, and is recognized as the cause of the low tem- 

 perature of the interior of a flame, and of the fact that the 

 flame forms a hollow cone of glowing gas. This driving away 

 of air occurs throughout a proportionately small space only, 

 and on the outer margin of this space the chemical combina- 

 tions constituting combustion take place. These facts are 

 so elementary that it would have been superfluous to men- 

 tion them, were it not that Benevides has constructed a 

 theory without taking them into consideration. From the 

 following passage one would derive a singular idea of the 

 nature of flame; for if the phrase "Taction mecanique du gaz 

 * Ann. de CJiim. et de Phys. [4] xxviii. 358. 



C2 



