350 Prof. W. M. Thornton on Ignition of Gases at 



that might give rise to singularities of the kind observed, the 

 inference is that there is a change of type of ion, from 

 clusters of molecules to the electronic state for example, or 

 of numbers of ions, which retards ignition, finally in the 

 case of. methane, partially in hydrogen and the other 

 paraffins, and gives rise to the point of inflexion in coal-gas. 

 The evidence of the next two sections supports the view that 

 any such critical points depend more on group formation 

 than upon duration of spark, though in ignition by transient 

 arcs the latter undoubtedly has great influence. But the 

 first and most decisive condition is that the temperature of 

 the point from which the spark starts should be sufficient to 

 permit discharge of ions from the heated surface under the 

 momentary high voltage gradient, and this discharge appears 

 to have a critical value at *4 of an atmosphere. 



6. Hydrogen. 



The points on the curve of fig. 1 are the results of 

 many observations of explosions. There are in the curve 

 five distinct stages. Here as in steady arc phenomena 

 atmospheric pressure is a point of transition. In passing 

 through it the current is doubled. There is from A to B no 

 change in the current. It then begins to rise, slowly at first, 

 suddenly at C, approaches another steady state and then 

 goes through a violent fluctuation in the neighbourhood of 

 04 atm., reaching the limit of inflammability at a little 

 over 03 of an atmosphere. This well-marked oscillation of 

 inflammability of hydrogen at low pressures has been pre- 

 viously observed in the ignition of coal gas by condenser 

 discharge sparks*, but the approach to it was then smooth. 

 It does not occur in hot-wire ignition f. The two steps from 

 A to B and C to D do not occur in break-spark ignition of 

 hydrogen. There is no sudden change of the chemical 

 or thermal properties of the mixture as a whole to account 

 for them. They appear to be a consequence of critical 

 relations between time of molecular contact in collision and 

 time of action of the spark, of the same nature as the steps 

 in methane. 



From the values of the ratio e/m clusters of molecules are 

 known to be formed around an electric charge and can to 

 some extent be identified. The groups change in size by the 

 successive loss of molecules, and it is in the integral relations 



* " The Reaction "between Gas and Pole/' he. cit. fig-. 5. 

 t "The Ignition of Gases by Hot Wires," Phil. Mag. vol. xxxviii. 

 p. 628 (Nov. 1919). 



