ON GASEOUS COMBUSTION. 473 



Ignition Temperature 

 under Adiabatic Compression 



Mixture Falk Dixon 



4H. 2 +0 2 605° — 



2H 2 +0o 540° 530° 



H.,+ 0," 514° 530° 



H,+ 20, 530° 520° 



H 2 +40 2 571° 507° 



Using the specific heats of hydrogen and air found by Joly under high 

 pressures, Dixon calculates the ignition-point of electrolytic gas to be 

 557° from his experiments on adiabatic compression. From Dixon's 

 results it would also appear that successive additions of oxygen to 

 electrolytic gas regularly lower the ignition-point. 



We are principally indebted to the photographic researches of H. B. 

 Dixon and his pupils 1 for the most notable recent additions to our know- 

 ledge concerning the initial phases of explosion and the phenomena 

 associated with the setting up of ' detonation ' in gaseous mixtures. The 

 first investigators to use a photographic method were Mallard and 

 Le Ohatelier in their researches on the initial phases of gaseous explo- 

 sions. 2 They recorded the movement of the flame along a horizontal 

 glass tube on a sensitised plate moving vertically, thus obtaining a graph 

 compounded of the two velocities. Failing to obtain any satisfactory 

 records with such feebly luminous flames as those yielded by mixtures 

 either of carbon monoxide or of hydrogen with oxygen, they employed 

 mixtures of carbon disulphide with either oxygen or nitric oxide, which 

 they regarded as typical of all ' oxygen ' or ' air ' mixtures respectively. 



The behaviour of these mixtures was found to differ according as 

 they were ignited near (a) the open or (b) the closed end of a tube. In 

 the case of (a) the flame proceeded for some distance down the tube at 

 a practically uniform velocity, which is the true rate of propagation ' by 

 conduction'; with the mixture CS 2 + 6NO this state was succeeded by 

 an oscillatory period, the flame swinging backwards and forwards with 

 increasing amplitudes, and then either dying out altogether or giving rise 

 to ' detonation ' ; with the oxygen mixtures the initial period of uniform 

 slow velocity was shorter, and appeared to be abruptly succeeded by 

 detonation without the intervention of any oscillatory period. "When 

 the nitric oxide mixtures were ignited near the closed end of the tube, 

 the forward movement of the flame was uniformly accelerated until 

 detonation was set up. 



According to Le Chatelier the following numbers represent in metres 

 per second the true initial rates of inflammation (i.e., of the propagation 

 of the flames ' by conduction ') for various mixtures : — 



Hydrogen and Air. 

 Hydrogen, per cent, 10 20 30 40 50 60 70 



Metres, per sec. . . . 060 1-95 3-30 4-37 3-45 2-30 1-10 



Methane and Air. 

 Methane, per cent. 6 8 10 12 14 16 



Metres, per sec. . . . 0-03 0'23 0-42 0-61 0-36 0-10 



1 Phil Trans., 1903, Series A, 200, 315. a Annales des Mines, 8< S6r.,4. 



