314 RESPIRATION 



is so impermeable to gas that it only adsorbs or gives off gas 

 very slowly. In the inflammable gas associated with oil fields 

 higher hydrocarbons are present in considerable amount, so that 

 the gas may burn with a luminous flame and has toxic properties. 

 Methane may of course also be produced by the action of bacteria 

 on old timber or other organic matter in the absence of oxygen ; 

 and accidents from the explosion of gas from this source have 

 occasionally occurred in British ironstone mines. 



When about 6 per cent of methane is present in air, the mixture 

 becomes inflammable with an ordinary light, and explodes vio- 

 lently with a somewhat higher percentage. Curiously enough, 

 however, an excess of methane prevents explosion, although plenty 

 of oxygen is still present; and with more than about 12 per cent 



10 



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Figure 74. 



Diagram showing outlines of caps visible on an oil flame with different 

 percentage of methane. 



of methane the mixture ceases to be inflammable. This fact limits 

 considerably the direct dangers from explosions of fire damp. 



The presence of nonexplosive proportions of fire damp in air 

 can easily be detected by the appearance of a "cap" on the flame 

 of a lamp. The cap is a pale, nonluminous flame which appears on 

 the top of the ordinary flame. In order to see it properly the ordi- 

 nary flame must be either effectively shaded or lowered till little 

 else than a blue flame is present, as otherwise the light from the 

 ordinary flame produces a dazzling effect which renders the cap 

 invisible, though it can be photographed without difficulty. The 

 length of the cap depends on the temperature and size of the 

 flame, and with the very hot hydrogen flame the test becomes far 

 more delicate, so that as little as 0.2 per cent of methane can be 

 detected easily. Figure 74 shows the outlines of the cap visible 



