356 Ignition of Gases at Reduced Pressures. 



point of sparking, than upon anything else ; in this resembling 

 the oscillation of hydrogen at the same pressure. The cause 

 o£ this has been discussed in § 8. 



10. Coal Gas. 



This is a mixture containing about 50 per cent, hydrogen, 

 35 per cent, methane, 10 to 15 per cent, carbon monoxide. 

 Its curve of ignition, fig. 6, shows clearly the influence of 

 hydrogen and methane but not of carbon monoxide. The 

 magnitude of the currents and the shape of the curve at 

 pressures not much below atmospheric resemble hydrogen. 

 The straight part of the carve is a mean line through 

 the methane steps. The point of inflexion occurs at the 

 same pressure as the oscillations in hydrogen and carbon 

 monoxide, and gives to the curve a form similar to that of the 

 pressure-volume curve of a gas at the critical temperature. 

 In the present case the analogue of temperature is the reci- 

 procal of time of duration of the igniting impulse. The 

 curve is of interest in showing the modification of the 

 hydrogen type by methane, though the gas in all essentials 

 behaves as hydrogen. 



11. Conclusions. 



Ignition by impulsive sparks is on the whole more difficult 

 at low pressures, in this differing from ignition by hot wires. 

 In hydrogen and methane the increase of difficulty takes 

 place by well-marked steps or stages. In ethane and propane 

 these do not occur, but there is an oscillation of the curve, 

 found also in hydrogen and carbon monoxide. This appears 

 to be associated with the temperature of the sparking point 

 during the passage of the spark. It has been shown previously 

 that at pressures well above atmospheric sudden changes of 

 inflammability occur when ignition is by impulsive sparks or 

 condenser discharge. It is now found that there are such 

 steps at reduced pressures and that there is a close agreement 

 between the partial pressures corresponding to the points at 

 which oxygen has ratios to the mass of combustible gas 

 expressed by successive natural numbers, and the pressures 

 at which sudden changes of inflammability occur. 



