370 



TRANSACTIONS OF SECTION B. 



appreciable ' pre-flame period ' ; only in the fastest burning mixtures is this 

 period negligible, and hence the necessity of stopping the movement of the 

 piston artificially at the beginning of the period, a precaution which Falk seems 

 to have neglected. 



According to Dixon and Croft's recent determination by this method of the 

 igiution-pomts of mixtures containing electrolytic gas, whereas successive 

 additions of hydrogen or nitrogen progressively raise the ignition-temperature 

 ot the undiluted gas by regular increments, as would be supposed, successive 

 additions of oxygen, on the other hand, lower it, as a glance at the followincr 

 table will show : 



The Irjmtwn-jwinf^ of Mixtures containhuj Elect luhjtic Gas by Adiabatic 



Compression. 



By H. B. Dixon and J. M. Crofts, 1914. 



Electrolytic Gas, 2H.2 + 02 = 526° 



+ a;H2 



x=l . . . 544° 



x=2 . . . 561° 



x= 4: . . . 602° 



a; = 8 . . . 676° 

 (526 + 182-)° 



+ X-N.2 



x= 1 



x=2 

 .r = 4 

 a;= 8 



(526+lla;)o 



537° 

 549° 



571° 

 015° 



+ a;0.2 



x^ 1 . 

 x= 7 . 

 a; = 15 . 



511° 



478° 

 472° 



The observed raising effects of successive dilutions with hydrogen and 

 nitrogen call for no comment, save that the relative greater effect of hydrogen, 

 as compared with nitrogen, may be attributed to its greater thermal con- 

 ductivity ; but the lowering effect of oxygen is indeed puzzling, and its 

 meaning can only be conjectured. Dixon and Crofts have suggested that it may 

 be due either to the formation of some active polymeride of oxygen under the 

 experimental conditions, which seems to me doubtful, or that the concentration 

 of oxygen in some way or other brings about increased ionisation of the com- 

 bustible gas. This at once raises the larger question of whether or not ignition 

 is a purely thermal problem, as until recently has generally been supposed. 



Professor W. M. Thornton, of Newcastle, has recently published some very 

 suggestive work on the Electrical Ignition of Gaseous Mixtures,* which, quite 

 apart from its theoretical interest, has an important bearing on the safety of 

 coal-mines where electrical currents are used for signalling and other purposes. 



The common belief that any visible spark will ignite a given explosive 

 mixture of gas and air is, of course, quite erroneous, for, just as Coward and 

 his co-workers have shown that for a given explosive mixture and sparking 

 arrangement there is a certain limiting pressure of the gaseous mixture below 

 which ignition will not take place, so from Thornton's work would it appear 

 that a definite minimum of circuit energy is required before a given mixture 

 at given pressure can be ignited by a spark. And, moreover, he has stated 

 that the circuit energy required for the spark ignition of a given mixture say of 

 methane and air is something like 56 times greater with alternating than with 

 continuous cm-rent at the same voltage. From this he has argued that the 

 igniting effect cannot be simply thermal, but must be in part at least ionic. 

 This conclusion he has further supported with the statement that the igniting 

 power of a unidirectional current is, in fact, proportional to the current in the 

 case of many gaseous mixtures over an important part of their working range 

 of inflammability. 



While there is much that is suggestive in Thornton's work, there is also a 

 good deal which seems very difficult to interpret from a chemical standpoint ; 1 

 refer more particularly to his later supposition of ' stepped ignition,' which is 

 based upon certain observed abrupt increases in the minimum igniting current 

 required with condenser discharge sparks as the proportion of combustible gas 



' Proc. Roy. Sac., Sec. A, vol. 90 (1914), p. 272; ibid. vol. 91 (1914), p. 17 



