20 Dr. Karl Heum aim's Contributions to 



sur la flamme" &c. be not taken in a figurative sense, Bene- 

 vides appears to regard the flame as a separate substance which 

 is carried along by the stream of gas: — " Lorsque Ton intro- 

 duit un solide, par exemple un fil metallique, on oppose une 

 resistance au mouvement du gaz, dont la vitesse diminue et 

 par consequent, Paction mecanique du gaz sur la flamme qui 

 tend a la projeter a distance diminue aussi, d'ou il resulte que 

 l'espace obscur diminue, et le jet lumineux se rapproche du 

 chalumeau." 



In opposition to this theory it must also be remembered 

 that extinction of the flame could not be caused by the gas- 

 stream driving back the air, because combustion would always 

 be possible at the line of contact between gas and air. Out- 

 ward and inward diffusion would continuously tend to increase 

 the magnitude of the space where combustion was possible. 

 It is therefore quite impossible that the space noticed by Bene- 

 vides between the burner and the flame of compressed coal- 

 gas could be caused by the absence of oxygen, the oxygen 

 having been driven away by the stream of issuing gas. By 

 this removal of oxygen the inner cold portion of the flame 

 would be increased in size; and the flame itself would be 

 lengthened by the increased velocity of the gas-stream ; but 

 extinction could not be brought about at the outer limits of the 

 flame-mantle, as was noticed by Benevides. 



The mechanical action of a rapid stream of gas upon the air 

 would also only cause an increase in the size of the flame, but 

 no removal of that flame from the burner. I believe that one 

 cause of this removal is to be found in the absorption of heat 

 occasioned by the gas issuing with so considerable a velocity, 

 but that a second cause is also at work, viz. the relation be- 

 tween velocity of the gas-stream and velocity of propagation 

 of combustion — a circumstance which Benevides overlooked in 

 his theory, although he had apparently noticed it in his expe- 

 riments. 



The cooling action exercised upon the lowest portion of the 

 flame by the quick inrush of gas may be divided into two 

 parts. The temperature of the cylindrical flame-layer formed 

 nearest to the burner is lowered by the coal-gas in the same 

 manner, although to a smaller degree (on account of the low 

 conductivity of gas for heat), as when a metallic rod is held in 

 the flame. 



The innermost portion of the burning layer, consisting of 

 coal-gas and air which has diffused inwards, may by this means 

 be cooled to such an extent as to be extinguished; in other 

 words, the ignited layer may be carried further from the point 

 where the gas issues, and an unburned mixture of gas and air 

 may take its place. 



