Gases at Reduced Pressures by Transient Arcs. 459 



latter is losing influence, because presumably of the 'cup and 

 ball ' action of an alternating field on ions from the arc. 

 Though the frequency is low it appears to be sufficiently high 

 to check the activity of the arc in promoting ignition. When 

 in the case of methane the current is continuous 0'5 of an 

 ampere at 200 volts will cause ignition, while a current 

 alternating at 100 periods a second requires 20 amperes to 

 be broken (with bright coruscations) before the gas explodes. 

 In the present case the change is not so great. Fig. 1 bears 

 to fig. 2 much the same resemblance as fig. 6 to fig. -7. The 

 position of the maximum at "65 atmosphere in the first is at 

 •4 in fig. G, and the minima have similarly moved. That is, 

 ionization now becomes critical at pressures lower than with 

 direct current. 



Methane has changed only in magnitude of the oscillations. 

 There is little sign of the ionization term here, the curve is 

 almost hyperbolic. The interesting point is that so small a 

 change of ordinate is significant. If fig. 7 had been observed 

 first the kink in it would have been possibly regarded as an 

 experimental error, instead of being as it is an indication of 

 a process reaching its maximum in fig. 1. 



The resemblance between the curve of change of state and 

 those of figs. 1 to 4 has been mentioned previously. Figs. 8 

 and 9 are illustrations of the flat stage corresponding to 

 liquefaction at constant pressure. All that is wanted to 

 complete the series is a curve having a point of inflexion 

 corresponding to a critical isothermal. This has been 

 observed witli disruptive spark ignition*, and with break 

 sparks at a frequency of GO when the gas is methane f. 



The meaning of a flat stage between two phases is that as 

 the influence of one falls it is exactly compensated by the 

 increase of the other. The cause of the fall of the thermal 

 term is the smaller number of molecules in unit volume: the 

 rise of the ionization term is the increase in velocity due to 

 the longer free path. It is clear that when these are the two 

 chief factors such a compensation is not improbable. 



8. Coal Gas and Carbon Monoxide, 



The ignition of these by continuous currents is given in 

 figs. 10 and 11 and by alternating currents in figs. 12 and 13. 

 Fig. 10 is a fair average of figs. 1 and 2. The magnitudes 

 of the ordinates are those of hydrogen and this is again found 

 by a comparison of figs. G and 12. As might have been 

 expected hydrogen is the cause of the more sensitive ignition 



* Proc. Roy. Soc. A, vol. xcii. 1015, figs. 6 and 7. 

 t Proc. Roy. Soc. A, vol. xc. 1914. fi_-. 9. 

 2H2 



