024 Prof. W. M. Thornton on the 



hydrogen until the wires are red-hot, the current then being 

 nearly 2 amperes as against 1*2 when nncoated. After several 

 explosions the salt is driven off, the metal surface is seen, 

 though it is not so bright, and the igniting current returns 

 to 1*2 amperes. Ignition does not depend therefore upon 

 the emission of positive ions alone, but upon some action 

 upon the combustible molecule by contact with and possible 

 penetration of the incandescent layer. This support; the 

 suggestion of Sir J. J. Thomson that u the action of surfaces 

 might ultimately be found to depend on the fact that they 

 formed a support for layers of electrified gas in which 

 chemical changes proceeded with high velocity." (Brit. 

 Assoc. Discussion loc. cit.) 



Hauser holds the view that explosion of mixtures of methane 

 by hot platinum is made by contact with gas burning at the 

 surface. Couriot and Meunier, on the other hand, regard 

 ignition as entirely due to the heat from the wire. 



The temperature of ignition is certainly not the apparent 

 temperature of surface combustion, nor is it that of the wire 

 that starts it, though both take part in the action. 



10. Influence of the Nature and Proportion of Combustible 

 Gas in the Mixtures. 



Comparison of the values of the coefficient b in Tables II. 

 and III. leads to the conclusion that the chemical nature of 

 the combustible molecule has little influence on hot-wire 

 ignition by tungsten or molybdenum, and that with platinum 

 b approaches a. steady value when the molecular weight is 

 about 60. That is, hydrogen atoms cease to have any dis- 

 tinctive influence on platinum ignition of hydrocarbons 

 above this point. 



The proportion of combustible gas has in general little 

 effect on the igniting currents : those of coal gas, fig. 4, are 

 sensiblj the same over the whole range of ignition. 



Table IV. 

 Methane. Tungsten Wires. 



Percentage of Igniting 



Gas in Air. Current. 



(V7 o"05 amperes. 



8-0 5-20 



10*0 5-25 



12-0 5-30 



13-5 5-30 



