PRESIDENTIAL ADDRESS. 



373 



otherwise — particularly with the more rapidly moving flames — vibrations may be 

 set up from the beginning. 



Whilst all methane-air mixtures develop an initial uniform slow flame- 

 movement period when ignited at or near the open end of a horizontal tube, 

 both its linear duration as well as the flame velocity are not, according to 

 private information which Dr. Wheeler has sent me, independent of the dimen- 

 sions of the tube {vide fig. 2). The speed of flame increases with the diameter 

 of the tube, and the linear duration of the uniform period increases with both 

 the diameter and length of the tube up to a certain maximum, after whdch 

 increase in length probably makes no appreciable difference ; also, for the same 

 tube, it varies with the proportion of methane in the explosive mixture, being 

 greater as the speed of the flame diminishes, until with the two ' limiting ' 

 explosive mixtures i't appears to last almost indefinitely. 



Dr. Wheeler's recent redetermination of the velocities of the flame move- 



Rate of Inflammation. 



{Le Chatelier.) 



Methane and Air. 



5 6 7 8 9 10 II 12 13 14 15 16 17 



% METHANE 



Fia. 1. 



ment during this initial uniform period for mixtures of methane and air in 

 varying proportions within the limits of inflammability has revealed serious 

 errors in Mallard and Le Chatelier's original results for horizontal tubes of the 

 same diameter as those which Dr. Wheeler has employed. Moreover, Mallard 

 and Le Chatelier's method of determining the composition of the upper and 

 lower limits of inflammability by extrapolation from their curves has been 

 proved to be unwarranted. Dr. Wheeler considers that the length of the tubes 

 used by Mallard and Le Chatelier (1 metre only) was insufficient to ensure that 

 the speed measurements of its initial uniform flame-movement period were 

 unaffected by the subsequent 'vibratory period.' Also, the methane used by 

 them, prepared as it was from sodium acetate, would obviously be impure. 



The most important differences between the latest results published by 

 Dr. Wheeler and those originally determined by Mallard and Le Chatelier, as 

 shown on the accompanying diagram (fig. 3) are as follows : 



(1) According to Wheeler, the limits of inflammability for horizontal propa- 

 gation of flame in methane-air mixtures, at atmospheric temperature and pres- 

 sure, correspond to 5-4 and U-'.i per cent, methane contents, respectively, 

 whereas Mallard and Le Chatelier gave 56 and 167 per cent. 



