348 Prof. W. M. Thornton on Ignition oj Gases at 



atmosphere. In the present case it was never obtained 

 below 0-25 atmosphere. In vacuum-tubes the temperature 

 of the bright bands of the striated positive column is about 

 100° C, too low in any case to ignite gas. At some point 

 about an eighth of an atmosphere ignition appears to fail r 

 but the fact that with different kinds of ignition the lower 

 limiting pressure may change from 0*13 to 0*4 of an atmo- 

 sphere shows that the argument of a simple thermal limit to 

 ignition will not hold. That there is thermal effect cannot 

 be doubted, and the hyperbolic form of some of the curves- 

 suggests that in certain cases it may be more marked than 

 others, but the singular variations, from which no mixtures 

 are free, show clearly that there are reactions other than 

 thermal between the spark and gas which profoundly modify 

 the process of ignition, especially of the simpler gases such as- 

 hydrogen and methane. 



5. General nature of results. 



The curves of figs. 1 to 6 have as ordinates the magnitude of 

 the primary current which when broken gives a secondary 

 spark just causing ignition. 



Considering figs. 1 to 6 as a whole, and remembering 

 that the secondary voltage of a transformer having an open 



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magnetic circuit and carrying a small current is proportional 

 to this primary current, the first observation is that as the 

 gas pressure is lowered difficulty of ignition rises, on account 



