Iron Vapour in Air-Coal Gas Flame. 231 



Secondly, there may be present in both the air-hydrogen 

 and air-acetylene cones too great a quantity of free hydrogen 

 and, as Messrs. Beilby and Henderson have shown, excess of 

 hydrogen is unfavourable to the formation of nitrides. 



Thirdly, the formation of the nitrides may be facilitated 

 by the presence of methane, or its reaction with oxygen, 

 acting as a catalytic agent. 



The first two of these explanations seem to me the most 

 plausible ones, and they appear even to lend support to the 

 hypothesis of the formation of nitrides in the air-coal gas 

 cone. Whether and in how far the other constituents of 

 coal gas, such as carbon monoxide, ethylene, &c, which are 

 present only in small proportions, participate in the reaction, 

 and also the exact role played by methane, are problems 

 which will require separate investigation. 



Until experimental evidence to the contrary is brought 

 forth I will assume, as a working hypothesis, that the emis- 

 sion of the characteristic cone spectrum of iron, composed of 

 lines of Classes II. and III., is the result of a reaction be- 

 tween nitrogen and the metal, consisting in the formation 

 and, perhaps also, subsequent decomposition of a nitride 

 of iron. 



That the nitride formed is probably an unstable one may 

 be conjectured from the fact that the material, almost as 

 soon as formed, passes into regions of rapidly increasing 

 temperatures. There are no observations which might pro- 

 vide an indication as to whether the decomposition of the 

 nitride takes place within the explosion region or only after 

 the material has passed into the mantle. It will be in- 

 teresting to know this, for if decomposition took place only 

 in the mantle this would prove that the decomposition of the 

 nitride, like that of the chloride, oxide, sulphate, (fee, is not 

 accompanied by the cone emission, but only by that of the 

 temperature lines. 



§ 6. Reason for the Appearance of Cone Lines near 

 the Base of the Oxy-coal Gas Flame. 



It is now possible to offer a satisfactory explanation for 

 the appearance at the base of the oxy-coal gas cone of traces 

 of Cass III. lines and also for the slight enhancement of 

 Class II. lines, referred to above (§ 2, b). Since the flame 

 burns in air, some of the latter is naturally drawn in at the 

 base and the nitrogen of this air is thus brought into contact 

 with the iron compound within the explosion region. Its 

 restriction to the base may be accounted for by assuming 



